We looked at calculating and using moles for solids using mass in unit 1.7. Here we are using moles of a solute dissolved in a solution and moles of a gas to carry out calculations.

Part of

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

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

Divide volumes in cm^{3} by 1000 to convert it into dm^{3}.

Calculating concentration

- Question
0.5 mol of solute is dissolved in 250 cm

^{3}of solution. Calculate its concentration.250 cm

^{3}= 250 ÷ 1000 = 0.25 dm^{3}Concentration = 0.5 ÷ 0.25 = 2.0 mol/dm

^{3}

Rearranging the equation: **amount = concentration × volume**

- Question
Calculate the amount of solute dissolved in 2 dm

^{3}of a 0.1 mol/dm^{3}solution.Amount = 0.1 × 2 = 0.2 mol

Rearranging the equation: **volume = amount ÷ concentration**

- Question
Calculate the volume of a 2 mol/dm

^{3}solution that contains 0.5 mol of solute.Volume = 0.5 ÷ 2 = 0.25 dm

^{3}Note that 0.25 dm

^{3}is the same as 250 cm^{3}(0.25 × 1000).

Concentrations are also expressed in grams per cubic decimeter (g/dm^{3}). Use therelative formula mass (M_{r}) to convert the two units of concentration.