pH of strong acids and bases
Strong acids and bases dissociate completely, so the concentration of H₃O⁺ or OH⁻ equals the concentration of the acid or base.
Formulas and rules
- pH = −log c(H₃O⁺)
- pOH = −log c(OH⁻)
- At 25 °C: pH + pOH = 14
- Strong acids: HCl, HNO₃, H₂SO₄ (first step); strong bases: NaOH, KOH
- Diluted tenfold → pH changes by 1
Your turn
Three problems on this topic. Type numbers like 12.5 or 1,250 and elasticities with their sign.
Task
Work out the pH of hydrochloric acid (HCl) with c = 0.02 mol/L (strong acid, fully dissociated).
pH (2 decimal places)
Numbers like 12.5 or 0.025, with as many decimals as asked. Masses and amounts count within 1%, pH values within ±0.02.
Worked example
Work out the pH of nitric acid (HNO₃) with c = 0.25 mol/L (strong acid, fully dissociated).
Worked solution
- 1.Strong acid: c(H₃O⁺) = c(acid) = 0.25 mol/L
- 2.pH = −lg c(H₃O⁺) = −lg(0.25) = 0.60
Typical mistake: Forgetting the minus in front of the log: lg(0.01) is −2, but the pH is +2.
Work out pOH and pH of sodium hydroxide solution (NaOH) with c = 0.025 mol/L at 25 °C (strong base).
Worked solution
- 1.c(OH⁻) = 0.025 mol/L → pOH = −lg(0.025) = 1.60
- 2.At 25 °C, pH + pOH = 14 → pH = 14 − 1.60 = 12.40
Typical mistake: Taking −lg c of a base as the pH. That is the pOH; the pH is 14 − pOH.
Common questions
Why is the pH lower at higher acid concentration?
Because of the minus in front of the log: more H₃O⁺ means a lower pH.