Stoichiometry Tutorial
Finding and Using Molar Ratios

The coefficients in a balanced chemical equation can be used to determine the relative number of molecules, formula units, or moles of a compound involved in a chemical reaction.

Example:

N2 (g) + 3 H2 (g) --> 2 NH3 (g)
1 molecule of nitrogen (N2) reacts with 3 molecules of hydrogen (H2) to form 2 molecules of ammonia (NH3)
OR
1 mole of nitrogen (N2) reacts with 3 moles of hydrogen (H2) to form 2 moles of ammonia (NH3)
The coefficients in a balanced equation can be used to write a molar ratio. Molar ratios are conversion factors that can be used to relate:
  1. moles of product formed from a certain number of moles of reactant
  2. moles of reactant needed to form a certain number of moles of a product.
  3. the number of moles of a particular reactant needed to completely react with a certain number of moles of a second reactant.
For the following reaction:
4 NH3 (g) + 5 O2 (g) --> 4 NO (g) + 6 H2O (g)
the following molar ratios can be written:

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4 moles NH3
5 moles O2
4 moles NH3
4 moles NO
4 moles NH3
6 moles H2O
5 moles O2
4 moles NO
5 moles O2
6 moles H2O
4 moles NO
6 moles H2O
The inverses of each of these molar ratios can also be written.
Example: What is the molar ratio between Li and N2 in the following reaction:
6 Li (s) + N2 (g) --> 2 Li3N (s)

 

Answer:
6 moles Li
1 mole N2
1 mole N2
6 moles Li
OR

Using molar ratios in calculations:

Molar ratios are used as conversion factors to relate the number of moles of one substance with the number of moles of another substance. The following example illustrates the steps used in any mole-to-mole conversion problem.
Example: Using the equation given in the previous example, use the following steps to determine the number of moles of Li3N that can be prepared using 0.24 mol Li.
  1. Write down the units you are looking for, including the formula of the compound, and an equals sign:

    mol Li3N

    =

       
  1. Write the number of moles you were given in the problem

    mol Li3N = 0.24 mol Li

  1. Put a multiplication sign after the number of moles and draw a line:
  2. mol Li3N = 0.24 mol Li x ____________
  1. Above and below the line, write the molar ratio. Since you want to get rid of the units you started with (in this case "mol Li), put the units "mol" and the formula of the compound you started with on the bottom. Put the units "mol" and the formula of the compound you're looking for on the top.
    mol Li3N = 0.24 mol Li x ___mol Li3N
    mol Li
  1. Use the coefficients from the balanced chemical equation to complete the molar ratio. Place the coefficient in front of the formula that you started with on the bottom and the coefficient in front of the formula that you are looking for on the top.

    6 Li (s) + N2 (g) --> 2 Li3N (s)

    mol Li3N = 0.24 mol Li x 2 mol Li3N
    6 mol Li
  1. Cancel out the units and do the math.

    mol Li3N = 0.24 mol Li x 2 mol Li3N = 0.080 mol Li3N
    6 mol Li
 

Practice Problems:

Use the following equation to answer the questions below:

2 CH3OH (l) + 3 O2 (g) --> 2 CO2 (g) + 4 H2O (l)

  1. How many moles of water will be produced from the combustion of 0.27 moles of CH3OH?
  2. How many moles of O2 are needed to burn 2.56 moles of CH3OH?
  3. How many moles of CO2 are produced from the combustion of 5.25 moles of CH3OH?
  4. How many moles of water are produced when 3.25 moles of CO2 are formed?

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