Titration and pH Lab

How to use it

1. Pick a strong (hydrochloric) or weak (acetic) acid. The flask holds 25.0 mL.
2. Set the concentrations, then add NaOH with the slider or the +0.1 mL and +1 mL buttons.
3. Read the pH meter and watch the flask colour. The shaded band on the graph is where your indicator changes colour.
4. The red dashed line marks the equivalence point, where moles of base equal moles of acid.

Try this

1. With 0.10 M acetic acid and 0.10 M NaOH, where is the equivalence point? Explain it with moles.
2. Find the half-equivalence point for acetic acid. What is the pH there, and why does it equal the pKa?
3. Compare the pH at equivalence for the strong acid and the weak acid. Why is one of them above 7?
4. Which indicator would you choose for each titration, and why would methyl orange be a poor choice for acetic acid?

The science

At the equivalence point, moles of NaOH added equal moles of acid in the flask: Veq = (C acid × V acid) ÷ C base.

For a weak acid, the region before equivalence is a buffer. Half-way to equivalence, [HA] = [A⁻], so pH = pKa (4.76 for acetic acid).

The lab calculates pH exactly from the charge balance, including water's own ionisation (Kw = 1.0 × 10⁻¹⁴ at 25 °C), so the curve is smooth through the equivalence point.

An indicator is useful only if its colour-change range sits inside the steep part of the curve. Phenolphthalein (8.2–10.0) suits a weak acid titrated with a strong base.