Chemical Calculations

Chemical calculations are essential for understanding what’s happening in reactions and for making sure we use the right amounts of substances. Whether you're working out how much product you’ll get or checking the efficiency of a process, these skills are key.

Calculations on this page:
Relative Formula Mass, Percentage of Mass, Concentration, Atom Economy, Percentage Yield

Relative Formula Mass (Mr)

The relative formula mass is the sum of the relative atomic masses (Ar) of all the atoms in a compound. You use this to work out masses in calculations.

Example:
Mr of NaCl = 23 (Na) + 35.5 (Cl) = 58.5

Percentage Mass of an Element in a Compound

This tells you how much of a compound’s mass is made up of a specific element.

Formula:
% Mass = (Ar × number of atoms of element ÷ Mr of compound) × 100

The Mole and the Number of Moles

A mole is a way of counting particles. One mole contains 6.02 × 10²³ particles (Avogadro’s number).

Formula:
Moles = Mass (g) ÷ Mr

This helps you calculate how many particles are in a given mass of substance.

Conservation of Mass

In chemical reactions, no atoms are lost or gained — the total mass of the products equals the mass of the reactants. If this doesn’t seem to happen, it’s often because a gas has entered or escaped from the system.

The Mole and Equations

You can use moles to calculate how much of each substance is involved in a reaction. Balanced equations tell you the ratio of moles for each reactant and product.

Example:
2H₂ + O₂ → 2H₂O means 2 moles of hydrogen react with 1 mole of oxygen.

Balancing Equations

Balanced equations are needed for all chemical calculations. They show the number of atoms and moles of each substance taking part in the reaction.

Limiting Reactants

The limiting reactant is the substance that runs out first in a reaction and stops more product from being made. You can identify it by calculating which reactant produces the least product.

Calculations Involving Gases

When gases are produced or used in reactions, you can calculate how much gas is involved using this equation (at room temperature and pressure):

Volume (dm³) = Moles × 24

Calculations Involving Solutions

Solutions have a concentration, which tells you how much solute is dissolved in a certain volume of solvent.

Concentration (g/dm³) = Mass ÷ Volume
or
Concentration (mol/dm³) = Moles ÷ Volume (dm³)

You may need to rearrange this to calculate moles or volume.

Titration and Concentration Calculations

In titration, you react an acid with an alkali to find out an unknown concentration. After collecting accurate volume readings, you can use the mole formula and a balanced equation to calculate the missing value.

Atom Economy

Atom economy is a measure of how much useful product is made in a reaction. It’s important for reducing waste and improving sustainability.

Formula:
Atom Economy = (Mr of desired products ÷ Mr of all products) × 100

Percentage Yield

Percentage yield tells you how much product you actually got compared to what you expected.

Formula:
% Yield = (Actual yield ÷ Theoretical yield) × 100

This is useful in industry to check if a process is efficient or needs improving.

Also see Rates of Chemical Change, Energy Changes, Electrolysis

Links for learning

BBC Bitesize: Relative formula mass
Study Mind: Relative formula mass
Free Science Lessons: Percentage by mass worksheet
BBC Bitesize: Mole calculations (higher)
MME: Calculating Moles
BBC Bitesize: Calculating concentration
MME: Calculating concetration