Mole calculations in solutions - Higher tier

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.

concentration~in~mol/dm^3 = \frac{moles}{volume~in~dm^3}

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

Divide volumes in cm3 by 1000 to convert it into dm3.

Calculating concentration

Question

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

250 cm3 = 250 ÷ 1000 = 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 × 1000).

Concentrations are also expressed in grams per cubic decimeter (g/dm3). Use therelative formula mass (Mr) to convert the two units of concentration.

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