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: 2H 2 + O 2  2H 2 O – Let’s say we may have 5.0 moles of H 2 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 H 2 + O 2  2 H 2 O 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 H 2 + O 2 → 2 H 2 O 2 molecules1 molecule2 molecules 2 mole1 mole2 mole

Example

Example continued – How many moles of water can be produced from the reaction of 5.0 moles of H 2 ?

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(NO 3 ) Na 3 PO 4  Ca 3 (PO 4 ) NaNO 3 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 moles of calcium phosphate?

Pause and Practice Answers

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