How are mole ratios used in chemical calculations?

May 11, 2014

In order to determine the mole ratio, we need to begin with a balanced chemical equation.

For this reaction the balanced chemical equation is
${N}_{2} + 3 {H}_{2} \to 2 N {H}_{3}$

The mole ratios are determined using the coefficients of the substances in the balanced chemical equation.

$1 m o l {n}_{2} : 3 m o l {H}_{2} : 2 m o l N {H}_{3}$

The mole ratios for each substance pair would be

$1 m o l {N}_{2} : 3 m o l {H}_{2}$

$\frac{1 m o l {N}_{2}}{3 m o l {H}_{2}}$ or $\frac{3 m o l {H}_{2}}{1 m o l {N}_{2}}$

$1 m o l {N}_{2} : 2 m o l N {H}_{3}$

$\frac{1 m o l {N}_{2}}{2 m o l N {H}_{3}}$ or $\frac{2 m o l N {H}_{3}}{1 m o l {N}_{2}}$

$3 m o l {H}_{2} : 2 m o l N {H}_{3}$

$\frac{3 m o l {H}_{2}}{2 m o l N {H}_{3}}$ or $\frac{2 m o l N {H}_{3}}{3 m o l {H}_{2}}$

Mole ratios are used as a means of comparison of substances in a balanced chemical equation in order to determine amounts.

Using the same reaction for ammonia
${N}_{2} + 3 {H}_{2} \to 2 N {H}_{3}$

How many moles of Hydrogen gas are necessary to react with 5 moles of Nitrogen.

We can use conversion factors in a process called stoichiometry.

$5 m o l {N}_{2} x \frac{3 m o l {H}_{2}}{1 m o l {N}_{2}} = 15 m o l {H}_{2}$

Mole ratio provides a comparison to cancel units. In this case the moles of Nitrogen cancel and we determine the moles of Hydrogen.