Mole calculations in solutions [Higher tier – GCSE Chemistry only]

Concentration calculations

You can calculate the amount of a substance in moles in a solution if you know the volume and concentration. You can also work out the concentration of an acid reacting with an alkali, or vice versa.

Concentration, amount of solute and volume of solution are linked by this equation:

Concentration in mol/dm3 = amount in mol ÷ volume in dm3

This equation can be rearranged to find the amount of solute or volume of solution.

Divide a volume in cm3 by 1,000 to convert it to a volume in dm3.

A calculation triangle with 'n (mol)' at the top and 'conc (mol/dm3)' and 'vol (dm3)' along the bottom.

Calculating concentration

Question

0.5 mol of solute is dissolved in 250 cm3 of solution. Calculate its concentration.

250 cm3 = 250 ÷ 1,000 = 0.25 dm3

Concentration = 0.5 ÷ 0.25 = 2.0 mol/dm3

Calculating amount of solute

Rearranging the equation: amount = concentration × volume

Question

Calculate the amount of solute dissolved in 2 dm3 of a 0.1 mol/dm3 solution.

Amount = 0.1 × 2 = 0.2 mol

Calculating volume

Rearranging the equation: volume = amount ÷ concentration

Question

Calculate the volume of a 2 mol/dm3 solution that contains 0.5 mol of solute.

Volume = 0.5 ÷ 2 = 0.25 dm3

Note that 0.25 dm3 is the same as 250 cm3 (0.25 × 1,000).

Concentrations can also be expressed in grams per decimetre cubed (g/dm3). To convert the two different units of concentration, use the relative formula mass (Mr).

Multiply by Mr to convert mol/dm3 to g/dm3. Divide by Mr to convert g/dm3 to mol/dm3.