Today S Activity Will Introduce You to a Unit of Measurement Without Which Chemistry Would

The Mole

Today’s activity will introduce you to a unit of measurement without which chemistry would not exist . It is a unit much like a dozen, which helps us count things. “Why would a chemist need to count things?” you might ask. Examine the following chemical reaction:

2 H2 + O2  2 H2O

This can be interpreted as two molecules of hydrogen and one molecule of oxygen combining to form two water molecules. But how often do we as chemists limit our reactions to one or two molecules? Usually a reaction is done on a billion, trillion scale, where an unimaginable number of molecules are involved. How then do we know we have the right mix? We need to count our molecules. Which brings us back to the Mole.

We will start by investigating two egg farmers (a chicken farmer and a quail farmer). They produce such large numbers of eggs that they can’t count them all, so they use dozens in some cases, in other cases they use mass (weighing is often easier than counting).

Although eggs vary in size, shape and mass we can use average values and get a general idea. Fill in the following table using the information below.

1 chicken egg = 37.44 g 1 dozen chicken eggs = 12 eggs 1 quail egg = 2.34 g 1 dozen quail eggs = 12 eggs

Chicken Quail Number of Mass of the Number of Mass of the Ratio of Ratio of masses eggs in the sample eggs in the sample numbers of of eggs sample sample eggs (chicken:quail) (chicken:quail) 1 37.44 g 1 2.34 g 1:1 = 1 16.0:1 = 16.0 10 10 438 438 1 dozen 1 dozen 12 dozen 12 dozen 1 million 1 million

1) How may times bigger in mass is a chicken egg than a quail egg?

2) What conclusion can you draw about the masses of two samples of eggs when the number of eggs in those samples are equal? Explain mathematically why this is so.

3) If the farmers preferred to use pounds instead of grams to weight their eggs would #2 still hold true?

4) If the farmers made up a new counting unit, like the “cluckster”, and you had two samples (3 clucksters of chicken eggs and 3 clucksters of quail eggs), what could you say about the ratio of their masses? Does it matter in this problem what a “cluckster” is?

- 1 - Let’s take what we learned in the egg model and apply it to atoms. Like eggs, atoms of the same kind may have different masses (isotopes). But the Periodic Table lists an average atomic mass for each type of atom. Keep in mind that the masses on the Periodic Table can be used as “atomic mass units” (amu or u), where 1 amu is approximately equal to the mass of a proton or neutron. To find the mass of a compound we add up the masses of the atoms in that compound. Here are some examples:

Mass of 1 Atom H 1.01 amu O 16.00 amu C 12.01 amu Na 22.99 amu Cu 63.55 amu

From these atomic masses, we can derive masses of combinations of atoms (called formula masses for ionic compounds and molecular masses for molecules):

Masses of combinations of atoms (i.e. one formula unit or one molecule) =

Mass of 1 Formula Unit Mass of 1 Molecule

NaOH 40.00 amu H2O 18.02 amu

Na2CO3 105.99 amu CO2 44.01 amu

CuO 79.55 amu O2 32.00 amu

NaHCO3 84.01 amu H2 2.02 amu

CH4 16.05 amu

5) Take a moment to check the values in the second table. How were they calculated?

Fill in the following table:

Sulfur Oxygen Number of Mass of the Number of Mass of the Ratio of Ratio of masses atoms in the sample atoms in the sample numbers of (S:O) sample sample atoms (S:O) 1 32.06 amu 1 16.00 amu 1:1 = 1 2.004:1 = 2.004 10 10 438 438 1 dozen 1 dozen 12 dozen 12 dozen 1 million 1 million 1 mole 32.06 g 1 mole 16.00 g 1:1 = 1 32.06 g:16.00 g = 2.004

6) Even though you are not sure “how many” a mole is, did you have problems filling in the table above? Why or why not?

7) If we were to change the mass unit we used to grams, what could you say about the relative number of atoms of oxygen in a 16.00 g sample, and the number of atoms of sulfur in a 32.06 g sample?

8) In the front of the room there is a bottle which contains a 32.06 g sample of sulfur. How many atoms do you think are in this bottle?

- 2 - A long time ago chemists discovered what you just discovered. If you use relative masses of samples, you’ll have the same number of atoms of each type. Since the Periodic Table already had relative masses listed in atomic mass units, they decided to develop a new unit – the Mole – which was a sample whose mass was its relative mass in grams. Below are the molar masses of the same substances we looked at on the previous page.

Mass of 1 Mole of Atoms = H 1.01 g O 16.00 g C 12.01 g Na 22.99 g Cu 63.55 g

Mass of 1 Mole of Each Substance =

NaOH 40.00 g H2O 18.02 g

Na2CO3 105.99 g CO2 44.01 g

CuO 79.55 g O2 32.00 g

NaHCO3 84.01 g H2 2.02 g

CH4 16.05 g

9) Compare these tables with the similar tables on the last page. What changed?

10) What can you conclude about the numbers of atoms in a 63.55 g sample of copper and a 22.99 g sample of sodium?

here for now.

So how many particles are in a mole? By estimating the size of atoms, and taking volume measurements of 1 mole samples it has been estimated that 1 mole = 6.022 x 1023 particles or 602,200,000,000,000,000,000,000 particles (more than you can count in a lifetime)!!

11) Use what you have learned to fill in the blanks:

1 atom of helium = ______amu He

1 molecule of chlorine (Cl2) = ______amu Cl2

1 formula unit of sodium chloride (NaCl) = ______amu NaCl

1 mole of helium = ______atoms of He = ______g He

1 mole of chlorine molecules = ______molecules of Cl2 = ______g Cl2

1 mole of sodium chloride = ______formula units of NaCl = ______g NaCl

1 mole of chlorine molecules (Cl2) = ______atoms of Cl