pH of a strong acid
pH = -log10[H+] (strong monoprotic acid: [H+] = concentration of acid)
For a strong monoprotic acid [H+] equals the acid concentration — then it's just -log.
Work it, then mark it
Do each calculation on paper first, then reveal the mark scheme and tick the marks you actually earned — the same way you should mark past papers.
Calculate the pH of 0.050 mol/dm3 hydrochloric acid. Give your answer to 2 decimal places.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Forgetting the minus sign in pH = -log10[H+].
- Assuming [H+] equals the concentration for a WEAK acid — that only holds for strong acids.
- Giving pH to the wrong precision — quote pH to 2 decimal places.
Exam tip: Only a STRONG monoprotic acid has [H+] equal to its concentration. A fully dissociating diprotic acid doubles [H+] (H2SO4 is usually treated this way as an approximation — its second ionisation isn't quite complete); a weak acid needs Ka.
Calculations are the most trainable marks in chemistry
They come up every paper and reward a clean method. Send me one you keep dropping marks on — your first lesson is free.