2. Purpose
• When performing chemical reactions, one needs to know how many
molecules (or moles) of a substance they are working with. For example,
if we have a reaction where we want to react 0.1 moles of a substance, we
need to be able to measure this out. We can do this using the average
atomic masses from the periodic table.
3. Average atomic mass is grams per mole
• Recall that average atomic mass on the periodic table is grams per mole
(g/mol).
– e.g. the mass of carbon is 12.011 g/mol
• This means 1 mole of carbon atoms has a mass of 12.011 grams.
• We can represent this relationship as a conversion factor for
dimensional analysis:
12.011 𝑔 𝐶
1 𝑚𝑜𝑙 𝐶
𝑜𝑟
1 𝑚𝑜𝑙 𝐶
12.011 𝑔 𝐶
𝑜𝑟
12.011 𝑔 𝐶
6.022 𝑥 1023 𝐶 𝑎𝑡𝑜𝑚𝑠
𝑜𝑟
6.022 𝑥 1023
𝐶 𝑎𝑡𝑜𝑚𝑠
12.011 𝑔 𝐶
Note: Since 1 mol of atoms is 6.022 x 1023 atoms, we could make the substitution in
for the molar mass.
4. Calculations with molar mass use dimensional
analysis
• When one is converting between mass, moles and atoms or molecules,
one will use dimensional analysis.
• Example:
– How many moles of carbon atoms are in 15.3 g of carbon?
• Given: 15.3 g C
• Asked for: moles of C
• Relationship: 1 mol = 12.011g
• Solve using dimensional analysis
15.3 𝑔 𝐶
1 𝑚𝑜𝑙 𝐶
12.011 𝑔 𝐶
= 1.27 𝑚𝑜𝑙 𝐶
Note: Recall that the mass comes straight from the periodic table, and for grams to
cancel out it must be in the denominator of the conversion factor.
5. Pause and Practice
• Convert the following using the masses from the periodic table: (show
your work and remember to use units)
– 1.23 moles of sodium to mass of sodium
– 45.6 g of iron to moles of iron
– 4.56 x 1022 atoms of silicon to grams of silicon
– 0.42 g of helium to atoms of helium
6. Pause and Practice Answers
• 1.23 moles of sodium to mass of sodium
1.23 𝑚𝑜𝑙 𝑁𝑎
22.99 𝑔 𝑁𝑎
1 𝑚𝑜𝑙 𝑁𝑎
= 28.3 𝑔 𝑁𝑎
• 45.6 g of iron to moles of iron
45.6 𝑔 𝐹𝑒
1 𝑚𝑜𝑙 𝐹𝑒
55.85 𝑔 𝐹𝑒
= 0.816 𝑚𝑜𝑙 𝐹𝑒
• 4.56 x 1022 atoms of silicon to grams of silicon
4.56 𝑥 1022
𝑎𝑡𝑜𝑚𝑠 𝑆𝑖
28.09 𝑔 𝑆𝑖
6.022 𝑥 1023 𝑎𝑡𝑜𝑚𝑠 𝑆𝑖
= 2.13 𝑔 𝑆𝑖
• 0.42 g of helium to atoms of helium
0.42 𝑔 𝐻𝑒
6.022 𝑥 1023 𝑎𝑡𝑜𝑚𝑠 𝐻𝑒
4.00 𝑔 𝐻𝑒
= 6.32 𝑥 1022
7. Try the Exercises
• Take your time with the conversion factor. You will have two of the
following:
– 1 mole
– the mass from the periodic table in grams
– 6.022 x 1023 atoms
