Stoichiometry

Purpose

• Stoichiometry is the process of mathematically relating quantities of chemicals in a reaction. The purpose of being able to do this is evident from a simple example:

2H2 + O2 2H2O

– Let’s say we may have 5.0 moles of H2 available for the reaction and want to know how much oxygen we need to react with it. Stoichiometry is the process that we will use to solve this.

Key Knowledge

• The keys to stoichiometry are the coefficients in the balanced chemical equation:

2 H2 + O2 2 H2O• The coefficients in the balanced equations can be thought of in terms of

molecules or moles:

• You can use these relationships in dimensional analysis to make conversions.

2 H2 + O2 → 2 H2O

2 molecules 1 molecule 2 molecules

2 mole 1 mole 2 mole

Example

• How many moles of O2 will react with 5.0 moles of H2?

2 H2 + O2 2 H2O

– From the balanced equation, 2 moles of H2 react with 1 mole of O2.

– We set-up the conversion with our given 5.0 moles of H2 and multiply it by the ratio of moles of O2 to moles of H2:

2.5 moles of O2 will react with 5.0 moles of H2.

• We can take this further and ask how much water will be made from the reaction?

Example continued – How many moles of water can be produced from the reaction of 5.0 moles of H2?

• How many moles of H2O will be produced if 5.0 moles of H2 completely reacts?

2 H2 + O2 2 H2O

– From the balanced equation, 2 moles of H2 will produce 2 moles of H2O.

– We set-up the conversion with our given 5.0 moles of H2 and multiply it by the ratio of moles of H2O to moles of H2:

5.0 moles of H2O will be produced from 5.0 moles of H2.

A few things to note from the example

• We always used the coefficients from the balanced equation for a mole ratio to convert from moles of one substance to moles of another.

• You can do this conversion from reactant to reactant, reactant to product, product to product and so on. As long as you have the balanced equation, you know the relationships.

• The conversion factor is set-up such that the given moles is canceled out by the moles in the denominator and the moles you are looking for is the new unit in the numerator of the conversion factor.

Pause and practice

3 Ca(NO3)2 + 2 Na3PO4 Ca3(PO4)2 + 6 NaNO3

• How many moles of calcium nitrate will react with 0.24 moles of sodium phosphate?

• How many moles of sodium nitrate will be produced from the complete reaction of 3.23 moles of calcium nitrate.

• How many moles of calcium nitrate are required to produce 0.063 moles of calcium phosphate?

Pause and Practice Answers

3 Ca(NO3)2 + 2 Na3PO4 Ca3(PO4)2 + 6 NaNO3

• How many moles of calcium nitrate will react with 0.24 moles of sodium phosphate?

• How many moles of sodium nitrate will be produced from the complete reaction of 3.23 moles of calcium nitrate.

• How many moles of calcium nitrate are required to produce 0.063 moles

of calcium phosphate?

Common mistake

• Be careful with the ratio to ensure that you have it set-up with the correct substances. If you write out your work, you will have a smaller chance of making mistakes.

Try the exercises