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Before this: Balancing chemical equations, Converting between mass and moles

Mole ratios in reactions

The coefficients in a balanced equation give the ratio in which substances react, counted in moles.

N2 + 3 H2 → 2 NH3

This says 1 mol of N2 reacts with 3 mol of H2 to make 2 mol of NH3. Any two substances in the equation give a mole ratio, which works as a conversion factor.

Using these ratios to calculate amounts is called stoichiometry.

Worked example: how many moles of NH3 form from 6.0 mol of H2?

The equation relates 3 mol of H2 to 2 mol of NH3.

6.0 mol H2 × (2 mol NH3 / 3 mol H2) = 4.0 mol NH3

The unit mol H2 cancels, leaving mol NH3.

Worked example: how many moles of N2 react with 6.0 mol of H2?

6.0 mol H2 × (1 mol N2 / 3 mol H2) = 2.0 mol N2

Common mistake

Using an unbalanced equation, or using the ratio upside down. Always balance first, then put the substance you want on top of the ratio.

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Watch What the coefficients mean
  1. 1 N2 reacts with 3 H2 to make 2 NH3.
  2. Start with twice as much N2 and everything doubles: 2 N2, 6 H2 and 4 NH3.
  3. The amounts changed but the ratio did not: 6 H2 to 4 NH3 is still 3 to 2.
  4. Count in moles instead of molecules and it is the worked example: 6 mol of H2 makes 4 mol of NH3.

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Practice

1. For 2 H2 + O2 → 2 H2O, how many moles of water form from 3.0 mol of O2?

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2. For N2 + 3 H2 → 2 NH3, how many moles of H2 are needed to make 10.0 mol of NH3?

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3. For 4 Fe + 3 O2 → 2 Fe2O3, how many moles of Fe2O3 form from 6.0 mol of Fe?

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4. For CH4 + 2 O2 → CO2 + 2 H2O, how many moles of O2 react with 2.5 mol of CH4?

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