All calculations
Kw and alkaline pH
Acids and bases
AQA 7405
ORIGINAL
Kw = [H+][OH-] = 1.00 x 10−14 mol2 dm−6 at 298 K
Convert pH to [H+] first, then use Kw to find [OH-].
Verified against AQA 7405 (2026 spec)
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.
Q1
3 marks
A solution has pH 11.40 at 298 K. Calculate its hydroxide-ion concentration. Kw = 1.00 x 10−14 mol2 dm−6.
Do the calculation on paper first — then mark it.
Where the marks get lost
- Using 14 - pH without checking the temperature is 298 K.
- Dividing [H+] by Kw rather than Kw by [H+].
Exam tip: At 298 K, pH + pOH = 14 is a shortcut, but the Kw method remains explicit and transferable.
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.