Home >  3-1 CHEM 1A: GENERAL CHEMISTRY Chapter 3: Stoichiometry of Formulas and Equations Instructor: Dr. Orlando E. Raola Santa Rosa Junior Col

# 3-1 CHEM 1A: GENERAL CHEMISTRY Chapter 3: Stoichiometry of Formulas and Equations Instructor: Dr. Orlando E. Raola Santa Rosa Junior Col

3-1

CHEM 1A: GENERAL  CHEMISTRY

Chapter 3:

Stoichiometry of Formulas and Equations

Instructor: Dr. Orlando E. Raola

Santa Rosa Junior College

3-2

Amount - Mass Relationships in Chemical Systems

3.5  Fundamentals of Solution Stoichiometry

3.1  The Mole

3.2  Determining the Formula of an Unknown Compound

3.3  Writing and Balancing Chemical Equations

3.4  Calculating Quantities of Reactant and Product

3-3

The mole (mol) is the amount of a substance that contains the same number of entities as there are atoms in exactly 0.012 kg of carbon-12.

One mole (1 mol) contains 6.022 �� 1023 entities
(to four significant figures).

This number is called Avogadro��s number and is abbreviated as NA.

The Mole

The term ��entities�� refers to atoms, ions, molecules, formula units, or electrons – in fact, any type of particle.

3-4

One mole (6.022x1023 entities) of some familiar substances.

Figure 3.1

3-5

The molar mass (M) of a substance is the mass per mole of its entites (atoms, molecules or formula units).

For monatomic elements, the molar mass is the same as the atomic mass in grams per mole. The atomic mass is simply read from the Periodic Table.

The molar mass of Ne = 20.18 g/mol.

Molar Mass

3-6

For molecular elements and for compounds, the formula is needed to determine the molar mass.

The molar mass of O2 = 2 x M of O

= 2 x 16.00

= 32.00 g/mol

The molar mass of SO2 = 1 x M of S + 2 x M of O

= 32.00 + 2(16.00)

= 64.00 g/mol

3-7

Table 3.1    Information Contained in the Chemical Formula of Glucose C6H12O6 ( M = 180.16 g/mol)

Carbon (C)

Hydrogen (H)

Oxygen (O)

Atoms/molecule of compound

6 atoms

12 atoms

6 atoms

Moles of atoms/mole of compound

6 mol of atoms

12 mol of atoms

6 mol of atoms

Atoms/mole of compound

6(6.022x1023) atoms

12(6.022x1023) atoms

6(6.022x1023) atoms

Mass/molecule of compound

6(12.01 u)

= 72.06 u

12(1.008 u)

= 12.10 u

6(16.00 u) = 96.00 u

Mass/mole of compound

72.06 g

12.10 g

96.00 g

3-8

Interconverting Moles, Mass, and Number of Chemical Entities

3-9

Mass-mole-number relationships for elements.

Figure 3.2

3-10

Sample Problem 3.1

Calculating the Mass of a Given Amount of an Element

PROBLEM:

SOLUTION:

amount (mol) of Ag

mass (g) of Ag

Silver (Ag) is used in jewelry and tableware but no longer in U.S. coins.  How many grams of Ag are in 0.0342 mol of Ag?

PLAN:

To convert mol of Ag to mass of Ag in g we need the molar mass of Ag.

multiply by M of Ag (107.9 g/mol)

0.0342 mol Ag x

1 mol Ag

107.9 g Ag

= 3.69 g Ag

3-11

Sample Problem 3.2

Calculating the Number of Entities in a Given Amount of an Element

PROBLEM:

Gallium (Ga) is a key element in solar panels, calculators and other light-sensitive electronic devices. How many Ga atoms are in 2.85 x 10-3 mol of gallium?

To convert mol of Ga to number of Ga atoms we need to use Avogadro��s number.

PLAN:

mol of Ga

atoms of Ga

multiply by 6.022x1023 atoms/mol

3-12

SOLUTION:

2.85 x 10-3 mol Ga atoms x

6.022x1023 Ga atoms

1 mol Ga atoms

= 1.72 x 1021 Ga atoms

Sample Problem 3.2

3-13

Sample Problem 3.3

Calculating the Number of Entities in a Given Mass of an Element

PROBLEM:

Iron (Fe) is the main component of steel and is therefore the most important metal in society; it is also essential in the body. How many Fe atoms are in 95.8 g of Fe?

The number of atoms cannot be calculated directly from the mass. We must first determine the number of moles of Fe atoms in the sample and then use Avogadro��s number.

PLAN:

mass (g) of Fe

amount (mol) of Fe

atoms of Fe

divide by M of Fe (55.85 g/mol)

multiply by 6.022x1023 atoms/mol

3-14

95.8 g Fe x

SOLUTION:

55.85 g Fe

1 mol Fe

= 1.72 mol Fe

1.72 mol Fe x

6.022x1023 atoms Fe

1 mol Fe

= 1.04 x 1024 atoms Fe

Sample Problem 3.3

3-15

Amount-mass-number relationships for compounds.

Figure 3.3

3-16

Sample Problem 3.4

Calculating the Number of Chemical Entities in a Given Mass of a Compound I

PROBLEM:

Nitrogen dioxide is a component of urban smog that forms from the gases in car exhausts. How many molecules are in 8.92 g of nitrogen dioxide?

number of  NO2 molecules

PLAN:

Write the formula for the compound and calculate its molar mass. Use the given mass to calculate first the number of moles and then the number of molecules.

divide by M

multiply by 6.022 x 1023 formula units/mol

mass (g) of NO2

amount (mol) of NO2

3-17

SOLUTION:

NO2 is the formula for nitrogen dioxide.

M = (1 x M of N) + (2 x M of O)

= 14.01 g/mol + 2(16.00 g/mol)

= 46.01 g/mol

8.92 g NO2

= 1.17 x 1023 molecules NO2

1 mol NO2

46.01 g NO2

6.022x1023 molecules NO2

1 mol NO2

= 0.194 mol NO2

0.194 mol NO

Sample Problem 3.4

3-18

Sample Problem 3.5

Calculating the Number of Chemical Entities in a Given Mass of a Compound II

PLAN:

PROBLEM:

Ammonium carbonate, a white solid that decomposes on warming, is an component of baking powder.

1. How many formula units are in 41.6 g of ammonium carbonate?

b) How many O atoms are in this sample?

Write the formula for the compound and calculate its molar mass. Use the given mass to calculate first the number of moles and then the number of formula units.

The number of O atoms can be determined using the formula and the number of formula units.

3-19

number of (NH4)2CO3 formula units

mass (g) of (NH4)2CO3

amount (mol) of (NH4)2CO3

SOLUTION:

(NH4)2CO3 is the formula for ammonium carbonate.

M = (2 x M of N) + (8 x M of H) + (1 x M of C) + (3 x M of O)

= (2 x 14.01 g/mol) + (8 x 1.008 g/mol)

+ (12.01 g/mol) + (3 x 16.00 g/mol)

= 96.09 g/mol

divide by M

multiply by 6.022 x 1023 formula units/mol

number of O atoms

3 O atoms per formula unit of (NH4)2CO3

Sample Problem 3.5

3-20

41.6 g (NH4)2CO

= 2.61x1023 formula units (NH4)2CO3

1 mol (NH4)2CO3

96.09 g (NH4)2CO3

6.022x1023 formula units (NH4)2CO3

1 mol (NH4)2CO3

= 0.433 mol (NH4)2CO3

0.433 mol (NH4)2CO3

2.61x1023 formula units (NH4)2CO3

3 O atoms

1 formula unit of (NH4)2CO3

=  7.83 x 1023 O atoms

Sample Problem 3.5

3-21

Mass % of element X =

atoms of X in formula �� atomic mass of X (u)

molecular (or formula) mass of compound (u)

x 100

Mass % of element X =

amount of X in formula (mol) �� molar mass of X (g/mol)

mass (g) of 1 mol of compound

x 100

Mass Percent from the Chemical Formula

3-22

Sample Problem 3.6

Calculating the Mass Percent of Each Element in a Compound from the Formula

PROBLEM:

Glucose (C6H12O6) is a key nutrient for generating chemical potential energy in biological systems. What is the mass percent of each element in glucose?

PLAN:

Find the molar mass of glucose, which is the mass of 1 mole of glucose. Find the mass of each element in 1 mole of glucose, using the molecular formula.

The mass % for each element is calculated by dividing the mass of that element in 1 mole of glucose by the total mass of 1 mole of glucose, multiplied by 100.

3-23

Sample Problem 3.6

multiply by M (g/mol) of X

divide by mass (g) of 1 mol of compound

multiply by 100

PLAN:

mass % X in compound

amount (mol) of element X in 1 mol compound

mass (g) of X in 1 mol of compound

mass fraction of X

3-24

6 mol C x

12.01 g C

1 mol C

= 72.06 g C

12 mol H x

1.008 g H

1 mol H

= 12.096 g H

6 mol O x

16.00 g O

1 mol O

= 96.00 g O

M = 180.16 g/mol

= 0.3999

mass percent of C =

72.06 g C

180.16 g glucose

x 100 = 39.99 mass %C

mass percent of H =

12.096 g H

180.16 g glucose

= 0.06714

x 100 = 6.714 mass %H

mass percent of O =

96.00 g O

180.16 g glucose

= 0.5329

x 100 = 53.29 mass %O

SOLUTION:

In 1 mole of glucose there are 6 moles of C, 12 moles H, and  6 moles O.

Sample Problem 3.6

3-25

Mass Percent and the Mass of an Element

Mass of element X present in sample =

mass of element in 1 mol of compound

mass of 1 mol of compound

mass of compound x

Mass percent can also be used to calculate the mass of a particular element in any mass of a compound.

3-26

Sample Problem 3.7

Calculating the Mass of an Element in a Compound

PLAN:

PROBLEM:

Use the information from Sample Problem 3.6 to determine the mass (g) of carbon in 16.55 g of glucose.

The mass percent of carbon in glucose gives us the relative mass of carbon in 1 mole of glucose. We can use this information to find the mass of carbon in any sample of glucose.

mass of glucose sample

mass of C in sample

multiply by mass percent of C in glucose

3-27

Mass (g) of C = mass (g) of glucose x

6 mol x M of C (g/mol)

mass (g) of 1 mol of glucose

SOLUTION:

Each mol of glucose contains 6 mol of C, or 72.06 g of C.

Sample Problem 3.7

= 16.55 g glucose x

72.06 g C

180.16 g glucose

= 6.620 g C

3-28

Empirical and Molecular Formulas

The empirical formula is the simplest formula for a compound that agrees with the elemental analysis. It shows the lowest whole number of moles and gives the relative number of atoms of each element present.

The empirical formula for hydrogen peroxide is HO.

The molecular formula shows the actual number of atoms of each element in a molecule of the compound.

The molecular formula for hydrogen peroxide is H2O2.

3-29

Sample Problem 3.8

Determining an Empirical Formula from Amounts of Elements

PROBLEM:

A sample of an unknown compound contains 0.21 mol of zinc, 0.14 mol of phosphorus, and 0.56 mol of oxygen. What is its empirical formula?

PLAN:

Find the relative number of moles of each element.  Divide by the lowest mol amount to find the relative mol ratios (empirical formula).

use # of moles as subscripts

amount (mol) of each element

preliminary formula

empirical formula

change to integer subscripts

3-30

Sample Problem 3.8

SOLUTION:

Using the numbers of moles of each element given, we write the preliminary formula Zn0.21P0.14O0.56

Next we divide each fraction by the smallest one; in this case 0.14:

0.21

0.14

= 1.5

0.14

0.14

= 1.0

0.56

0.14

= 4.0

We convert to whole numbers by multiplying by the smallest integer that gives whole numbers; in this case 2:

1.5 x 2 = 3

1.0 x 2 = 2

4.0 x 2 = 8

This gives us the empirical formula Zn3P2O8

This gives Zn1.5P1.0O4.0

3-31

Sample Problem 3.9

Determining an Empirical Formula from Masses of Elements

mass (g) of each element

amount (mol) of each element

PROBLEM:

Analysis of a sample of an ionic compound yields 2.82 g of Na, 4.35 g of Cl, and 7.83 g of O.  What is the empirical formula and the name of the compound?

preliminary formula

empirical formula

change to integer subscripts

use # of moles as subscripts

divide by M (g/mol)

PLAN:

Find the relative number of moles of each element.  Divide by the lowest mol amount to find the relative mol ratios (empirical formula).

3-32

Sample Problem 3.9

SOLUTION:

2.82 g Na x

1 mol Na

22.99 g Na

= 0.123 mol Na

4.35 g Cl x

1 mol Cl

35.45 g Cl

= 0.123 mol Cl

7.83 g O x

1 mol O

16.00 g O

= 0.489 mol O

The empirical formula is Na1Cl1O3.98  or NaClO4;

this compound is named sodium perchlorate.

0.123

0.123

0.489

0.123

Na and Cl =

= 1  and  O =

= 3.98

3-33

Determining the Molecular Formula

empirical formula mass (g/mol)

molar mass (g/mol)

= whole-number multiple

The molecular formula gives the actual numbers of moles of each element present in 1 mol of compound.

The molecular formula is a whole-number multiple of the empirical formula.

3-34

Sample Problem 3.10

Determining a Molecular Formula from Elemental Analysis and Molar Mass

PLAN:

assume 100 g lactic acid; then mass % = mass in grams

amount (mol) of each element

PROBLEM:

Elemental analysis of lactic acid (M = 90.08 g/mol) shows it contains 40.0 mass % C, 6.71 mass % H, and 53.3 mass % O. Determine the empirical formula and the molecular formula for lactic acid.

divide each mass by M

use # mols as subscripts; convert to integers

divide M by the molar mass for the empirical formula; multiply empirical formula by this number

empirical formula

molecular formula

3-35

SOLUTION:

Assuming there are 100. g of lactic acid;

40.0 g C x

1 mol C

12.01 g C

= 3.33 mol C

CH2

empirical formula

mass of CH2

molar mass of lactate

90.08 g/mol

30.03 g/mol

C3H6O3 is the molecular formula

O3.33

3.33

H6.66

3.33

C3.33

3.33

6.71 g H x

1 mol H

1.008 g H

= 6.66 mol H

1 mol O

16.00 g O

53.3 g O x

= 3.33 mol O

Sample Problem 3.10

= 3

3-36

Combustion apparatus for determining formulas
of organic compounds.

Figure 3.4

CnHm + (n+     ) O2   =  n CO(g) +      H2O(g)

m

2

m

2

3-37

Sample Problem 3.11

Determining a Molecular Formula from Combustion Analysis

PROBLEM:

When a 1.000 g sample of vitamin C (M = 176.12 g/mol) is placed in a combustion chamber and burned, the following data are obtained:

mass of CO2 absorber after combustion = 85.35 g

mass of CO2 absorber before combustion = 83.85 g

mass of H2O absorber after combustion = 37.96 g

mass of H2O absorber before combustion = 37.55 g

What is the molecular formula of vitamin C?

PLAN:

The masses of CO2 and H2O produced will give us the masses of C and H present in the original sample. From this we can determine the mass of O.

3-38

(mass after combustion – mass before) for each absorber

= mass of compound in each absorber

mass of each oxidized element

mass of O

molecular formula

Sample Problem 3.11

mass of each compound x mass % of oxidized element

divide each mass by M

mol of C, H, and O

use # mols as subscripts; convert to integers

mass of vitamin C – (mass of C + H)

empirical formula

3-39

SOLUTION:

For CO2: 85.35 g - 83.85 g = 1.50 g

1.50 g CO2 x

12.01 g C

44.01 g CO2

= 0.409 g C

Sample Problem 3.11

For H2O: 37.96 g - 37.55 g = 0.41 g

0.41 g H2O

2.016 g H

18.02 g H2O

= 0.046 g H

mass of O = mass of vitamin C – (mass of C + mass of H)

= 1.000 g - (0.409 + 0.046) g = 0.545 g O

3-40

176.12 g/mol

88.06 g

= 2.000 mol

= 0.0341 mol O

0.545 g O

16.00 g/mol O

0.409 g C

12.01 g/mol C

= 0.0341 mol C

0.046 g H

1.008 g/mol H

= 0.0456 mol H

Convert mass to moles:

Sample Problem 3.11

= 1

0.0341

0.0341

0.0456

0.0341

= 1.34

0.0341

0.0341

= 1

C1H1.34O1 = C3H4.01O3

C3H4O3

C6H8O6

Divide by smallest to get the preliminary formula:

Divide molar mass by mass of empirical formula:

3-41

Table 3.2   Some Compounds with Empirical Formula CH2O (Composition by Mass:  40.0% C, 6.71% H, 53.3% O)

Name

Molecular Formula

Whole-Number Multiple

M

(g/mol)

Use or Function

formaldehyde

acetic acid

lactic acid

erythrose

ribose

glucose

CH2

C2H4O2

C3H6O3

C4H8O4

C5H10O5

C6H12O6

30.03

60.05

90.09

120.10

150.13

180.16

disinfectant;  biological preservative

acetate polymers; vinegar (5% soln)

part of sugar metabolism

sour milk; forms in exercising muscle

component of nucleic acids and B2

major energy source of the cell

CH2

C2H4O2

C3H6O3

C4H8O4

C5H10O5

C6H12O6

3-42

Table 3.3   Two Pairs of Constitutional Isomers

Property

Ethanol

Dimethyl Ether

M (g/mol)

Boiling Point

Density at 200

Structural

formula

46.07

78.50

0.789 g/mL

(liquid)

46.07

-250

0.00195 g/mL

(gas)

Butane

2-Methylpropane

C4H10

C2H6

58.12

58.12

Space-filling

model

-0.50

0.579 g/mL

(gas)

-11.060

0.549 g/mL

(gas)

3-43

Chemical Equations

A chemical equation uses formulas to express the identities and quantities of substances involved in a physical or chemical change.

Figure 3.6

The formation of HF gas on the macroscopic and molecular levels.

3-44

A three-level view of the reaction between magnesium and oxygen.

Figure 3.7

3-45

Features of Chemical Equations

Mg       +         O2                 MgO

Reactants are written on the left.

A yield arrow points from reactants to products.

Products are written on the right.

The equation must be balanced; the same number and type of each atom must appear on both sides.

3-46

translate the statement

balance the atoms using coefficients;

formulas cannot be changed

specify states of matter

check that all atoms balance

Balancing a Chemical Equation

magnesium and oxygen gas react to give magnesium oxide:

Mg + O2 �� MgO

2Mg + O2 �� 2MgO

2Mg (s) + O2 (g) �� 2MgO (s)

3-47

translate the statement

Sample Problem 3.12

Balancing Chemical Equations

PROBLEM:

PLAN:

SOLUTION:

balance the atoms

specify states of matter

Within the cylinders of a car��s engine, the hydrocarbon octane (C8H18), one of many components of gasoline, mixes with oxygen from the air and burns to form carbon dioxide and water vapor.  Write a balanced equation for this reaction.

check the atoms balance

C8H18 +    O2           CO2 +    H2

2C8H18 + 25O2       16CO2 +  18H2

2C8H18 + 25O2       16CO2 +  18H2

2C8H18(l) + 25O2 (g)        16CO2 (g) +  18H2O (g)

C8H18 +        O2              8 CO2 +      9 H2

25

2

3-48

Molecular Scene

Combustion of Octane

3-49

Sample Problem 3.13

Balancing an Equation from a Molecular Scene

PROBLEM:

The following molecular scenes depict an important reaction in nitrogen chemistry. The blue spheres represent nitrogen while the red spheres represent oxygen.  Write a balanced equation for this reaction.

PLAN:

Determine the formulas of the reactants and products from their composition. Arrange this information in the correct equation format and balance correctly, including the states of matter.

3-50

SOLUTION:

The reactant circle shows only one type of molecule, composed of 2 N and 5 O atoms. The formula is thus N2O5. There are 4 N2O5 molecules depicted.

The product circle shows two types of molecule; one has 1 N and 2 O atoms while the other has 2 O atoms. The products are NO2 and O2. There are 8 NO2 molecules and 2 O2 molecules shown.

Sample Problem 3.13

The reaction depicted is 4 N2O5 �� 8 NO2 + 2 O2

Writing the equation with the smallest whole-number coefficients and states of matter included;

2 N2O5 (g) �� 4 NO2 (g) + O2 (g)

3-51

Stoichiometric Calculations

• The coefficients in a balanced chemical equation
• represent the relative number of reactant and product particles
• and the relative number of moles of each.
• Since moles are related to mass
• the equation can be used to calculate masses of reactants and/or products for a given reaction.
• The mole ratios from the balanced equation are used as conversion factors.

3-52

Table 3.4   Information Contained in a Balanced Equation

Viewed in Terms of

Products
3 CO2(g) +   4 H2O(g

Amount (mol)

Mass (amu)

3 molecules CO2 + 4 molecules H2

3 mol CO2  +   4 mol H2

132.03 amu CO2 +  72.06 amu H2

Reactants
C3H8(g) +   5 O2(g

Molecules

1 molecule C3H8 + 5 molecules O2

1 mol C3H8  +   5 mol O2

44.09 amu C3H8 + 160.00 amu O2

Mass (g)

132.03 g CO2 +  72.06 g H2

44.09 g C3H8 + 160.00 g O2

Total Mass (g)

204.09 g

204.09 g

3-53

Summary of amount-mass-number relationships in a chemical equation.

Figure 3.8

3-54

Sample Problem 3.14

Calculating Quantities of Reactants and Products: Amount (mol) to Amount (mol)

PROBLEM:

Copper is obtained from copper(I) sulfide by roasting it in the presence of oxygen gas) to form powdered copper(I) oxide and gaseous sulfur dioxide.

How many moles of oxygen are required to roast 10.0 mol of copper(I) sulfide?

PLAN:

write and balance the equation

moles of oxygen

use the mole ratio as a conversion factor

SOLUTION:

2 Cu2S (s) + 3 O2 (g) �� 2 Cu2O (s) + 2 SO2 (g

= 15.0 mol O2

3 mol O2

2 mol Cu2

10.0 mol Cu2S x

3-55

Sample Problem 3.15

Calculating Quantities of Reactants and Products: Amount (mol) to Mass (g)

PROBLEM:

During the process of roasting copper(I) sulfide, how many grams of sulfur dioxide form when 10.0 mol of copper(I) sulfide reacts?

PLAN:

Using the balanced equation from the previous problem, we again use the mole ratio as a conversion factor.

use the mole ratio as a conversion factor

mol of copper(I) sulfide

mol of sulfur dioxide

mass of sulfur dioxide

multiply by M of sulfur dioxide

3-56

= 641 g SO2

10.0 mol Cu2S x

2 mol SO2

2 mol Cu2

SOLUTION:

2 Cu2S (s) + 3 O2 (g) �� 2 Cu2O (s) + 2 SO2 (g

Sample Problem 3.15

64.07 g SO2

1 mol SO2

x

3-57

Sample Problem 3.16

Calculating Quantities of Reactants and Products: Mass to Mass

PROBLEM:

During the roasting of copper(I) sulfide, how many kilograms of oxygen are required to form 2.86 kg of copper(I) oxide?

PLAN:

divide by M of oxygen

mass of oxygen

mol of oxygen

mass of copper(I) oxide

multiply by M of copper(I) oxide

mol of copper(I) oxide

use mole ratio as conversion factor

3-58

= 0.959 kg O2

= 20.0 mol Cu2

SOLUTION:

2 Cu2S (s) + 3 O2 (g) �� 2 Cu2O (s) + 2 SO2 (g

Sample Problem 3.16

2.86 kg Cu2O x

103

1 kg

1 mol Cu2

143.10 g Cu2

1 kg

103

20.0 mol Cu2O x

3 mol O2

2 mol Cu2

32.00 g O2

1 mol O2

x

3-59

Reactions in Sequence

• Reactions often occur in sequence.
• The product of one reaction becomes a reactant in the next.
• An overall reaction is written by combining the reactions;
• any substance that forms in one reaction and reacts in the next can be eliminated.

3-60

Sample Problem 3.17

Writing an Overall Equation for a Reaction Sequence

PROBLEM:

Roasting is the first step in extracting copper from chalcocite, the ore used in the previous problem.  In the next step, copper(I) oxide reacts with powdered carbon to yield copper metal and carbon monoxide gas.  Write a balanced overall equation for the two-step process.

PLAN:

Write individual balanced equations for each step.

Adjust the coefficients so that any common substances can be canceled.

3-61

SOLUTION:

Cu2O (s) + C (s) �� 2Cu (s) + CO (g

2Cu2S (s) + 3O2 (g) �� 2Cu2O (s) + 2SO2 (g

Write individual balanced equations for each step:

Adjust the coefficients so that the 2 moles of Cu2O formed in reaction 1 are used up in reaction 2:

2Cu2S (s) + 3O2 (g) �� 2Cu2O (s) + 2SO2 (g

2Cu2O (s) + 2C (s) �� 4Cu (s) + 2CO (g

Sample Problem 3.17

2Cu2S (s) + 3O2 (g)  + 2C (s) �� 2SO2 (g) + 4Cu (s) + 2CO (g)

3-62

Limiting Reactants

• So far we have assumed that reactants are present in the correct amounts to react completely.
• In reality, one reactant may limit the amount of product that can form.
• The limiting reactant will be completely used up in the reaction.
• The reactant that is not limiting is in excess – some of this reactant will be left over.

3-63

An ice cream sundae analogy for limiting reactions.

Figure 3.10

3-64

Sample Problem 3.18

Using Molecular Depictions in a Limiting-Reactant Problem

PROBLEM:

Chlorine trifluoride, an extremely reactive substance, is formed as a gas by the reaction of elemental chlorine and fluorine. The molecular scene shows a representative portion of the reaction mixture before the reaction starts. (Chlorine is green, and fluorine is yellow.)

1. Find the limiting reactant.
2. Write a reaction table for the process.
3. Draw a representative portion of the mixture after the reaction is complete. (Hint: The ClF3 molecule has 1 Cl atom bonded to 3 individual F atoms).

3-65

PLAN:

Write a balanced chemical equation.  To determine the limiting reactant, find the number of molecules of product that would form from the given numbers of molecules of each reactant. Use these numbers to write a reaction table and use the reaction table to draw the final reaction scene.

Sample Problem 3.18

SOLUTION:

The balanced equation is Cl2 (g) + 3F2 (g) �� 2ClF3 (g

There are 3 molecules of Cl2 and 6 molecules of F2 depicted:

= 6 molecules ClF3

2 molecules ClF3

1 molecule Cl2

3 molecules Cl2

= 4 molecules ClF3

2 molecules ClF3

3 molecule Cl2

6 molecules F2

Since the given amount of F2 can form less product, it is the limiting reactant.

3-66

Sample Problem 3.18

Molecules

Cl2 (g)    +

3F2 (g)   ��

2ClF3 (g)

Initial

Change

3

-2

6

-6

0

+4

Final

We use the amount of F2 to determine the ��change�� in the reaction table, since F2 is the limiting reactant:

The final reaction scene shows that all the F2 has reacted and that there is Cl2 left over. 4 molecules of ClF2 have formed:

3-67

Sample Problem 3.19

Calculating Quantities in a Limiting-Reactant Problem: Amount to Amount

PROBLEM:

In another preparation of ClF3, 0.750 mol of Cl2 reacts with 3.00 mol of F2.

1.   Find the limiting reactant.
2.   Write a reaction table.

PLAN:

Find the limiting reactant by calculating the amount (mol) of ClF3 that can be formed from each given amount of reactant. Use this information to construct a reaction table.

SOLUTION:

The balanced equation is Cl2 (g) + 3F2 (g) �� 2ClF3 (g

= 1.50 mol ClF3

2 mol ClF3

3 mol F2

3.00 mol F2  x

2 mol ClF3

1 mol Cl2

0.750 mol Cl2

= 2.00 mol ClF3

Cl2 is limiting, because it yields less ClF3.

3-68

Sample Problem 3.19

Moles

Cl2 (g)    +

3F2 (g)   ��

2ClF3 (g)

Initial

Change

0.750

-0.750

3.00

- 2.25

0

+1.50

Final

0.75

1.50

All the Cl2 reacts since this is the limiting reactant. For every 1 Cl2 that reacts, 3 F2 will react, so 3(0.750) or 2.25 moles of F2 reacts.

3-69

Sample Problem 3.20

Calculating Quantities in a Limiting-Reactant Problem: Mass to Mass

PROBLEM:

A fuel mixture used in the early days of rocketry consisted of two liquids, hydrazine (N2H4) and dinitrogen tetraoxide (N2O4), which ignite on contact to form nitrogen gas and water vapor.

PLAN:

Find the limiting reactant by calculating the amount (mol) of ClF3 that can be formed from each given mass of reactant. Use this information to construct a reaction table.

(a) How many grams of nitrogen gas form when 1.00 x 102 g of N2H4 and 2.00 x 102 g of N2O4 are mixed?

(b) Write a reaction table for this process.

3-70

Sample Problem 3.20

divide by M

mass (g) of N2H4

mol of N2H4

mol of N2

mole ratio

divide by M

mass (g) of N2O4

mol of N2O4

mol of N2

mole ratio

mass of N2

select lower number of moles of N2

multiply by M

3-71

3 mol N2

2 mol N2H4

3.12 mol N2H4

1.00x 102 g N2H4

1 mol N2H4

32.05 g N2H4

Sample Problem 3.20

For N2H4

= 3.12 mol N2H4

For N2O4

SOLUTION:

2N2H4 (l) + N2O (l) �� 3N2 (g) + 4H2O (g

2.17 mol N2O4

3 mol N2

1 mol N2O4

2.00x 102 g N2O4

1 mol N2O4

92.02 g N2O4

= 6.51 mol N2

4.68 mol N2

28.02 g N2

1 mol N2

= 131 g N2

= 4.68 mol N2

= 2.17 mol N2

N2H4 is limiting and only 4.68 mol of N2 can be produced:

3-72

Sample Problem 3.20

Moles

2N2H4 (l)   +

N2O4 (l)  ��

3N2 (g)   +

4H2O (g)

Initial

Change

3.12

-3.12

2.17

- 1.56

0

+4.68

0

+6.24

Final

0.61

4.68

6.24

All the N2H4 reacts since it is the limiting reactant. For every 2 moles of N2H4 that react 1 mol of N2O4 reacts and 3 mol of N2 form:

3.12 mol N2H4

1 mol N2O4

2 mol N2H4

= 1.56 mol N2O4 reacts

3-73

Reaction Yields

The actual yield is the amount of product actually obtained.

The actual yield is usually less than the theoretical yield.

The theoretical yield is the amount of product calculated using the molar ratios from the balanced equation.

theoretical yield

actual yield

% yield =

x 100

3-74

The effect of side reactions on the yield of the main product.

Figure 3.11

3-75

Sample Problem 3.21

Calculating Percent Yield

PROBLEM:

Silicon carbide (SiC) is made by reacting sand(silicon dioxide, SiO2) with powdered carbon at high temperature.  Carbon monoxide is also formed.  What is the percent yield if 51.4 kg of SiC is recovered from processing 100.0 kg of sand?

PLAN:

write balanced equation

find mol reactant

percent yield

find mol product

find g product predicted

3-76

x 100

= 77.0%

51.4 kg

66.73 kg

= 66.73 kg

1664 mol SiC x

40.10 g SiC

1 mol SiC

1 kg

103

mol SiO2 = mol SiC = 1664 mol SiC

= 1664 mol SiO2

103

1 kg

100.0 kg SiO2

1 mol SiO2

60.09 g SiO2

SOLUTION:

SiO2(s) + 3C(s)  ��  SiC(s) + 2CO(g

Sample Problem 3.21

3-77

Solution Stoichiometry

• Many reactions occur in solution.
• A solution consists of one or more solutes dissolved in a solvent.
• The amount concentration of a solution is given by the amount (mol)  of solute present in a given volume of solution (L).

3-78

Sample Problem 3.22

Calculating the Molarity of a Solution

PROBLEM:

What is the amount concentration of an aqueous solution that contains 0.715 mol of glycine (H2NCH2COOH) in 495 mL?

SOLUTION:

PLAN:

divide by volume in L

mol of glycine

concentration in mol/L

0.715 mol glycine

495 mL soln

1000 mL

1 L

= 1.44 mol/L glycine

3-79

Summary of mass-mole-number-volume relationships in solution.

Figure 3.13

3-80

Sample Problem 3.23

Calculating Mass of Solute in a Given Volume of Solution

PROBLEM:

How many grams of solute are in 1.75 L of 0.460 M sodium monohydrogen phosphate buffer solution?

volume of solution

moles of solute

grams of solute

multiply by M

multiply by M

SOLUTION:

PLAN:

Calculate the moles of solute using the given molarity and volume. Convert moles to mass using the molar mass of the solute.

1.75 L x

0.460 moles

1 L

= 0.805 mol Na2HPO4

0.805 mol Na2HPO4  x

141.96 g Na2HPO4

1 mol Na2HPO4

= 114 g  Na2HPO4

3-81

Figure 3.14

A

• Weigh the solid needed.
• Transfer the solid to a volumetric flask that contains about half the final volume of solvent.

B  Dissolve the solid thoroughly by swirling.

C  Add solvent until the solution reaches its final volume.

3-82

Converting a concentrated solution to a dilute solution.

Figure 3.15

3-83

Sample Problem 3.24

Preparing a Dilute Solution from a Concentrated Solution

PROBLEM:

��Isotonic saline�� is a 0.15 mol/L aqueous solution of NaCl.  How would you prepare 0.80 L of isotonic saline from a 6.0 mol/L stock solution?

PLAN:

To dilute a concentrated solution, we add only solvent, so the moles of solute are the same in both solutions. The volume and molarity of the dilute solution gives us the moles of solute. Then we calculate the volume of concentrated solution that contains the same number of moles.

volume of dilute soln

amount of of NaCl in dilute soln =
amount  NaCl in concentrated soln

V of concentrated soln

multiply by concentration of dilute soln

divide by concentration  of concentrated soln

3-84

Mdil x Vdil = # mol solute = Mconc x Vconc

SOLUTION:

= 0.020 L soln

0.12 mol NaCl x

1 L soln

6.0 mol NaCl

0.80 L soln x

0.15 mol NaCl

1 L soln

= 0.12 mol NaCl

Sample Problem 3.24

Using the volume and molarity for the dilute solution:

Using the moles of solute and molarity for the concentrated solution:

A 0.020 L portion of the concentrated solution must be diluted to a final volume of 0.80 L.

3-85

Sample Problem 3.25

Visualizing Changes in Concentration

PROBLEM:

The beaker and circle represents a unit volume of solution.  Draw the solution after each of these changes:

PLAN:

Only the volume of the solution changes; the total number of moles of solute remains the same. Find the new volume and calculate the number of moles of solute per unit volume.

(a) For every 1 mL of solution, 1 mL of solvent is added.

(b) One third of the volume of the solution is boiled off.

3-86

SOLUTION:

Ndil x Vdil  =  Nconc x Vconc

where N is the number of particles.

(a)

Ndil = Nconc

Vconc

Vdil

= 8 particles x             =  4 particles

1 mL

2 mL

Sample Problem 3.25

(b)

Nconc = Ndil

Vdil

Vconc

= 8 particles x                =  12 particles

1 mL

mL

(a)

(b)

3-87

Sample Problem 3.26

Calculating Quantities of Reactants and Products for a Reaction in Solution

PROBLEM:

A 0.10 mol/L HCl solution is used to simulate the acid concentration of the stomach.  What is the volume (L) of ��stomach acid�� react with a tablet containing 0.10 g of magnesium hydroxide?

PLAN:

Write a balanced equation and convert the mass of Mg(OH)2 to moles. Use the mole ratio to determine the moles of HCl, then convert to volume using molarity.

divide by M

mass Mg(OH)2

mol Mg(OH)2

use mole  ratio

divide by M

mol HCl

L HCl

3-88

SOLUTION:

0.10 g Mg(OH)2

1 mol Mg(OH)2

58.33 g Mg(OH)2

= 1.7x10-3 mol Mg(OH)2

= 3.4x10-3 mol HCl

3.4x10-3 mol HCl x

1L HCl soln

0.10 mol HCl

= 3.4x10-2 L HCl

Sample Problem 3.26

Mg(OH)2 (s) + 2HCl (aq) �� MgCl2 (aq) + 2H2O (l

2 mol HCl

1 mol Mg(OH)2

= 1.7x10-3 mol Mg(OH)2 x

3-89

Sample Problem 3.27

Solving Limiting-Reactant Problems for Reactions in Solution

PROBLEM:

In a simulation mercury removal from industrial wastewater, 0.050 L of 0.010 mol/L mercury(II) nitrate reacts with 0.020 L of 0.10 mol/L sodium sulfide.  What is the mass of mercury(II) sulfide that would form?

PLAN:

Write a balanced chemical reaction.  Determine limiting reactant.  Calculate the mass of mercury(II) sulfide product.

3-90

= 5.0x10-4 mol HgS

= 2.0x10-3 mol HgS

Hg(NO3)2 is the limiting reactant because it yields less HgS.

5.0 x 10-4 mol HgS x

232.7 g HgS

1 mol  HgS

= 0.12 g HgS

Sample Problem 3.27

SOLUTION:

Hg(NO3)2 (aq) + Na2S (aq) �� HgS (s) + 2NaNO3 (aq

0.050 L Hg(NO3)2

1 mol HgS

1 mol Hg(NO3)2

0.010 mol Hg(NO3)2

1 L Hg(NO3)2

0.020 L Na2S x

1 mol HgS

1 mol Na2

0. 10 mol Na2

1 L Na2

x

3-91

Amount

Hg(NO3)2 (aq) +

Na2S (aq)  ��

HgS (s)  +

2NaNO3 (aq)

Initial

Change

5.0 x 10-4

-5.0 x 10-4

2.0 x 10-3

-5.0 x 10-4

0

+5.0 x 10-4

0

+1.0 x 10-3

Final

1.5 x 10-3

5.0 x 10-4

+1.0 x 10-3

Sample Problem 3.27

The reaction table is constructed using the amount of Hg(NO3)2 to determine the changes, since it is the limiting reactant.

3-92

Figure 3.16

An overview of amount-mass-number stoichiometric relationships. 