## Stoichiometry Tutorial Using Molar Mass in Calculations

Molar Mass as a Conversion Factor

##### As you will recall, the molar mass of a compound tells you the mass present in 1 mole of that substance. In other words, it tells you the number of grams per mole of a compound. Since the molar mass relates two different sets of units, grams and moles, it can be used as a conversion factor. The easiest way to convert between grams and moles (or vice versa) is by using dimensional analysis....i.e. keeping track of your units as you go along.

Converting Grams A to Moles A

1. ##### Write down the units that you are looking for (including the formula for the compound) and an equal sign:
 mol NaCl =

2. ##### Write down the number of grams you were given in the problem:
 mol NaCl = 12.5 g NaCl

3. ##### Put a multiplication sign after the mass and draw a line
 mol NaCl = 12.5 g NaCl x ________

• ##### put "1 mol" on the top
 mol NaCl = 12.5 g NaCl x 1 mol NaCl 58.5 g NaCl

5. ##### Cancel out the "grams" and do the math.
 mol NaCl = 12.5 g NaCl x 1 mol NaCl = 0.214 mol NaCl 58.5 g NaCl

### Chem 1215

Converting Moles A to Grams A

1. ##### Write down the units that you are looking for (including the formula for the compound) and an equal sign:
 g NaCl =
2. ##### Write down the number of moles you were given in the problem:
 g NaCl = 0.122 mol NaCl
3. ##### Put a multiplication sign after the moles and draw a line
 g NaCl = 0.122 mol NaCl x ________
• ##### put "1 mol" on the bottom
 g NaCl = 0.122 molNaCl x 58.5 g NaCl 1 mol NaCl
5. ##### Cancel out the "mol" and do the math.
 g NaCl = 0.122 mol NaCl x 58.5 gNaCl = 7.14 g NaCl 1 mol NaCl

Practice Problems:

1. 12.5 g CaCl2 = _______ mol CaCl2
2. 10.0 g Na2SO4 = _______ mol Na2SO4
3. 16 g NaOH = _______ mol NaOH
4. 125 mg NaOH = ______ mol NaOH
5. 0.125 mol CaCl2 = _____ g CaCl2
6. 0.25 mol Na2SO4 = ______ g Na2SO4
7. 1.55 mol NaOH = ______ g NaOH