Strong acid calculations are really easy if you learn them the right way. This post gives you notes on how to do the calculations for A-level chemistry and includes worked examples for both monoprotic/monobasic and diprotic/dibasic strong acids.
Strong acid calculations for A level Chemistry
For all pH calculations, learn these equations and symbols:
pH = -log[H+ ] (remember to give pH to 2 d.p.) and [H+ ] = 10- pH
[H+ ] = the concentration of protons
[HA] = the concentration of the acid
[A- ] = the concentration of the conjugate base
Monoprotic strong acids: Fully dissociate in aqueous solution
∴ [H+ ] = [HA] + –
e.g. HCl → H + Cl
If the concentration of HCl is 0.5 mol dm- 3 the concentration of H+ will also be 0.5 mol dm -3.
How to calculate the pH of a strong acid:
Because [H +] = [HA], the calculation becomes pH = -log[HA]
e.g. Calculate the pH of a solution of HCl with a concentration of 0.250 mol dm -3
pH = -log 0.250 = 0.60 (2 dp)
How to calculate the concentration [HA] of a strong acid:
Because [H +] = [HA], the calculation becomes [ HA] = 10 -pH
e.g. Calculate the concentration of a solution of HCl with a pH of 0.89.
[HA] = 10- 0.89 = 0.129 mol dm -3
Diprotic Strong acids
Fully dissociate in aqueous solution. 1 mol of acid dissociates to give 2 moles of protons. ∴ [ H + ] = 2[HA]
e.g. H2SO4 → 2H + + SO42-
If the concentration of H2SO4 is 0.5 mol dm-3 the concentration of H+ will be
1.0 mol dm- 3 .
How to calculate the pH of a dibasic strong acid
pH = -log[H+ ] , so first you need to calculate [H + ]: [ H+ ] = 2 x [HA]
e.g Calculate the pH of a solution of H2 SO4 with a concentration of 0.400 mol dm- 3 [H+ ] = 2 x 0.400 = 0.800 mol dm -3
Then, pH = -log 0.800 = 0.10 (2 dp)
How to calculate the concentration of a dibasic strong acid:
You’ll be given pH, so calculate [H + ]: [H +] = 10– pH Then, since [H+ ] = 2[HA], [HA] = [H +] 2
e.g. Calculate the concentration of a solution of H2 SO4 w ith a pH of 0.75. [H+ ] = 10 -0.75 = 0.179 mol dm -3
Then, [HA] = 0.179 0.0895 mol dm- 3
Test your knowledge
Take the acids, bases and buffers multiple choice questions quiz to test your knowledge of the strong acid calculations for A level chemistry.