Stoichiometry Tutorial
Using Molar Mass in Calculations

Molar Mass as a Conversion Factor

Chemical reactions occur on a molecular (or atomic) basis. When we want to plan a chemical reaction, we must know the number of molecules (or atoms or formula units) of a compound that should be present in the flask. The easiest way to keep track of (or count) the number of molecules (etc) present is to use the mole. Although we always plan experiments using the number of moles of reactants and/or products, we commonly carry out those experiments by measuring out a specific mass of each reactant. Once we plan the experiment, we then need a way to convert between grams and moles of a particular substance.

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

   ________

   
   

4. Write the molar mass (including the units) above and below the line you've drawn in such a way that the grams cancel out.

   • put the number of grams in the molar mass on the bottom

   • 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

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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

   ________

4. Write the molar mass (including the units) above and below the line you've drawn in such a way that the moles cancel out.

   • put the number of grams in the molar mass on the top

   • 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