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Dear Faculty Advisors of Chemistry Teacher Candidates,

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Introducing the Mole

CRYSTAL

Lewthwaite

Introducing the Mole C30S-3-07

Part A: How are These Substances Different Yet Similar?

On the table are twelve chemicals.

Look at the chemicals. In what ways are they different? You should be able to identify a variety of ways in which they are different both in terms of physical and chemical properties

1.                                                                                     2.

3.                                                                                     4.

5.                                                                                     6.

7.                                                                                     8.

9.                                                                                     10.

Suggest reasons why they are different?

In what ways are these chemicals the same? Suggest ways at the molecular level in which they are the same. This is much more difficult but based on your previous study of atoms and elements you should be able to consider ways in which they are similar at the sub-microscopic level?

One way in which these chemicals are the same is that they are all made up protons, neutrons and electrons. Although they may have a different number of these subatomic particles, the nature of the subatomic particles that make up these substances is the same. For example, although sulfur and carbon are made up of atoms which in turn are composed or protons, neutrons and electrons the sub-atomic particles that compose the atoms of sulfur and carbon are the same.

As well, the same number of molecules, atoms or ionic units or particles that make up each of these chemicals is the same. As an example the number of particles that make up the 18 g water sample is the same as the number of molecules that make up the 342g sugar sample and the number of ionic units that make up the 58.5 g sodium chloride unit.

One Sugar Molecule   (C12H22O11)            Many Water Molecules (H2O)

This particle level is the level at which chemical reactions occur. Thus when zinc and sulfur react to form zinc sulfide, in theory, an equivalent number of these reacting particles are involved even though the one mole of sulfur has a mass of 32. 1 g and zinc has mass of 65.4 g. This particle quantity is large. This amount is 6.02 x 1023.

Zn     +        S                                                       ZnS

                           32. 1 g           65.4 g                                               97.5 g

We will spend some time developing an understanding of how large this number is and, at the same time, an idea of how small these reacting compositional units are.

One mole of high school textbooks would cover the Canada to a depth of about 320 km.

Part B: The Atomic Mass Unit

If these substances all are made up of the same number (a mole - 6.02 x 1023) of particles, how do we account for the fact that they have different masses? As an example how can a mole of water have a mass of 18 g, a mole of sodium chloride 58.5 g and a mole of sugar 342 g? Obviously, it has to do with the size of the particle that is the building block for each of these substances. A dozen grapefruits weigh much more than a dozen oranges because the individual units, the grapefruit and orange, are different in mass

Let��s look at this idea more closely. Using the three different seeds we have on display (two heavy and one light) make a Bohr model example of one of the first eighteen elements of the Periodic Table. Note that in making the model we will use the smallest seed to model the electron, the lightest of the sub-atomic particles.

Place the models on the floor and identify any patterns or trends associated with the models. Re-arrange the models according to these patterns in order to re-create the first elements of the Periodic Table.

Chemically, how do these atoms differ?

Physically, how do these atoms differ?

Explain why these atoms differ in mass.

Note that the atomic mass for each element is determined by the number of protons and neutrons in the atom, not the number of electrons. Simply put, the atomic mass is a measure of the number of atomic mass units (amu) an atom contains. For example, the atomic mass (A) of carbon is exactly 12.

The mass of a proton and neutron are equivalent and very, very small. The mass of each of these units is 1.66 x 10 -24 grams -  a mass so small it��s hard to conceptualize. The atomic mass unit is a measure of the mass of a neutron or proton.

The smallest atom is the Hydrogen atom. Unlike most atoms, the Hydrogen atom only has one proton and one electron. There is no neutron. The Hydrogen atom��s mass is determined by this one atomic mass unit.

What would the mass of one mole of Hydrogen atoms be??

Remember the electron has a negligible mass. Thus to determine the mass of a Hydrogen atom we need only the mass of a proton and the number of atoms in a mole:

1.66 x 10 -24  g is the mass of the proton.

Since a mole of Hydrogen atoms contains 6.02 x 1023  atoms we can multiply these numbers together to get the mass of a Hydrogen atom.

1.66 x 10 -24 g (mass of one proton) x 6.02 x 1023  atoms = .999 g or 1 g

Note that this is the approximate atomic mass of H on the Periodic Table.

Again, one Hydrogen atom contains one atomic mass unit (a proton) and one mole of Hydrogen atoms weighs one gram. One mole of atomic mass units has a mass of 1 g.

If we look at our constructed models, the number of atomic mass units (amu) will determine the mass of one mole (atomic mass) of the atom. One mole of carbon atoms with 12 atomic mass units (6 protons + 6 neutrons) has a mass of 12 g.

Part C: Determining the Mass of Mole Amounts

Can you now see why a mole amount of each one of these substances would have a different mass?

What would the mass of each of one mole (a molar mass or molecular mass) of each of these substances be? In determining the mass of compounds composed of atoms we need to consider the total number of amu of each atom in the compounds. As an example, one mole of sucrose (C12H22O11) with 12 C atoms (each containing 12 amu), 12 H atoms (each containing 12 amu) and 12 O (each containing 16 amu) will have a mass of 342 g.

The following chart should assist you in determining the mass of a molar quantity mass of each of these substances.

Begin by completing the chart below. Given the information provided you should be able to determine the mass of one mole of each of these atoms by knowing the number of atomic mass units in the atom.

Element

Number of Protons

Number of Electrons

Number of Neutrons

Atomic mass (total amu)

Mass of 1 mole of these atoms.

Hydrogen

1

0

0

1

1g

Carbon

6

6

6

12

Oxygen

8

16

16g

Sodium

23

23g

Magnesium

24

24g

Aluminum

13

14

Sulfur

32

32g

Chlorine

18

Cobalt

60

60g

Nickel

59

59g

Copper

29

64

64g

By using these atomic masses you can now determine the mass of one mole (molecular mass) of some of the substances on display and other common compounds.

Substance

Formula of Molecule or Ionic Unit

Atomic Composition

Total AMU

Molar Mass (Mass of One Mole)

Water

H20

2(H) + 1(O)

2(1) + 1(16)=18

18g

Sucrose

Sodium Chloride

Copper Sulfate

Cobalt Chloride

Chlorine gas

Methane

Aluminum Choride

Ethanol

Time to Check: Based on our study thus far:

    With reference to the structure of the atom, what is an atomic mass unit?
    With reference to the structure of the atom, what is meant by atomic mass?
    With reference to the composition of atoms that make up molecules, what is meant by molecular or molar mass?
    Explain why the mass of one mole of a molecule such as ethanol (C2H5OH) is different than that of ethane (C2H6)? Which has a greater molar mass and why?

Part D: Calculating Mole Quantities

In this final section we will determine the molar amount present in a sample.

At the table are a number of bags with different amounts of these same substances.

The mass of each is given. For each, try to determine how many moles of each are present in the sample. In other words, the sugar sample may weigh 110.12 g. How many moles is this? How many sugar molecules does it represent? Again, one mole is represented by 6.02 x 1023 particles.

Based on your previous calculations, how do we determine the number of moles each mass represents? Write in the mathematical operations that are required to determine the unknown value in the relationship triangle provided.

Substance

Molar Mass

(Total amu)

Total Mass of Substance

Number of  Moles of Substance

Number of Particles of Substance

sucrose

342 g/mol

110.12g

110.12g/342 g/mol = ___mol

Summary:

    A sample of sucrose weighs 750 g. How many moles of sucrose does this represent?
    What mass is represented by 0.20 moles of sodium chloride?
    A sample of water has a mass of 34.0 g? How many moles is this? How many molecules are contained within this sample?Chlorine gas has a molar mass of 71.0 g. What does this mean? Explain why it has a molar mass of 71.0 g.
    Calculate the molar mass of the following:

a) Propane

b) Glucose


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