Weak-Acid Titration: pK_a at Half-Equivalence

Weak acid + strong base. Find V_eq from the pH jump, then read pH at V_b = V_eq/2. That pH is pK_a.

School· 24 min·Related simulator: ChemistryTitration Simulator

Goal

Determine pK_a from Henderson–Hasselbalch: at half-equivalence [A⁻]=[HA], so pH = pK_a.

Equipment

Burette (known C_b)
Flask (known V_a, unknown HA)
pH meter
Universal indicator

Experiment

Weak acid + strong base. Live pH is the bench meter.

Known aliquot V_a = 25.0 mL of unknown weak acid (HA). Titrant is strong base (NaOH) at C_b = 0.10 mol/L. Acid concentration is hidden. Only the burette volume changes.

Burette volume V_bV_b = 0.0 mL

pH meter = 2.8

Record stores this V_b and a noised pH. Find the steep jump on the meter, then park at half that volume. The bench does not mark a special point.

Record at least 6 pH readings at the volume you chose. Processing uses the mean of the pH column.

Measurements

№	V_b mL	pH
No measurements yet — take your first reading.

Data processing

Add at least 2 trials to compute the sample mean.

Uncertainty

Lab report

Opens the system print dialog — choose “Save as PDF” or your printer. Header and footer are hidden when printing.

One click opens the print dialog — choose “Save as PDF”.

Weak-Acid Titration: pK_a at Half-Equivalence

Generated: 24 Aug 2026, 22:29

Goal

Determine pK_a from Henderson–Hasselbalch: at half-equivalence [A⁻]=[HA], so pH = pK_a.

Measurement table

#	V_b (mL)	pH
No measurements yet — take your first reading.

Fit and derived value

Add at least 2 trials to compute the sample mean.

Conclusion

The mean pH at half-equivalence agrees with the hidden pK_a. Main uncertainties: judging V_eq by eye, meter noise, and the ideal Henderson–Hasselbalch buffer.

PhysSandbox virtual lab — values come from your session; add your own discussion of error sources.

Theory

For a monoprotic weak acid titrated with strong base, the buffer region has pH = pK_a + log₁₀([A⁻]/[HA]). Halfway to equivalence, half the acid is neutralized, so the ratio is 1 and pH = pK_a. Equivalence itself is basic (the conjugate base hydrolyzes) — do not read pK_a there. The bench never prints pK_a, V_eq, or a Henderson hint.

Procedure

Aliquot V_a and titrant C_b are fixed and known. Acid concentration and pK_a are hidden. You only change the burette V_b.
Watch the live pH meter and indicator. Find the jump (equivalence), note that V_eq, then set V_b = V_eq/2.
Record. The row stores V_b and the pH. Small meter noise is added. The bench does not mark half-equivalence.
Repeat the half-equivalence reading at least 6 times (re-find the jump if you like).
Take the mean of the pH column. Compare with the reference pK_a.

Conclusion

The mean pH at half-equivalence agrees with the hidden pK_a. Main uncertainties: judging V_eq by eye, meter noise, and the ideal Henderson–Hasselbalch buffer.