Identify the unknown acid.

The learning goal

Students use a known strong base to estimate an unknown strong monoprotic acid concentration. They assemble the equipment, add measured doses, recognize a sharp pH change, bracket the equivalence point, and compare three fresh-sample trials.

This guided version automates mixing, pH sampling, preparation of repeat aliquots, and calculations. The student decisions are a prediction, observing the endpoint, repeating measurements, and explaining precision using evidence. Manual concentration calculation is an optional extension.

Prerequisites

Meaning of pH, acidic/basic solutions, neutralization, and concentration. Introduce the burette before beginning. This is a strong-acid/strong-base model at 25°C: pH 7 at equivalence is not a general rule for every titration.

Student procedure

1. Predict whether a more concentrated acid would require more or less base.
2. Place stand, burette, 20.0 mL acid sample, indicator, probe, and base in the guided order.
3. Read initial pH. Add 5.0 mL portions of 0.100 M NaOH until 10.0 mL has been added.
4. Continue with 0.2 mL portions. Identify the last acidic and first basic readings, or a neutral reading if one is obtained.
5. Use the interval midpoint as an endpoint estimate. Save and repeat with two fresh samples of the same unknown.
6. Compare endpoint estimates, average them, and interpret C_acid = C_baseV_base/V_acid.
7. Explain how smaller doses reduce the endpoint interval and cite a measured volume.

Expected results

A new investigation selects one model unknown: approximately 0.065, 0.075, or 0.085 M. At nominal 20.0 mL sample volume and 0.100 M base, corresponding ideal endpoints are 13.0, 15.0, and 17.0 mL. The selected unknown stays the same for all three repeat aliquots. “Try a new unknown” may randomly choose the same concentration again.

Example: 0.100 M × 15.0 mL ÷ 20.0 mL = 0.075 M. An estimated endpoint of 14.9 mL gives 0.0745 M, reported as approximately 0.075 M. Assess evidence and rounding, not exact agreement with the hidden value.

The measured pH can jump from well below 7 to well above 7 even with a 0.2 mL dose. Students do not need to “hit” exactly pH 7. A narrow bracket is usable evidence. If a measured reading is exactly 7.00, the app uses that observed volume as its estimate.

Suggested assessment · 12 points

3 = accurate and supported; 2 = mostly accurate but incomplete; 1 = a limited or partly incorrect explanation; 0 = absent. The automatic check evaluates completion and a precision choice; it does not grade the written science.

Discussion prompts

• Why is the 0.2 mL dose useful even though the probe shows two decimal places?
• Why is adding base not the same as diluting with water?
• What would happen to the endpoint volume if the acid sample volume doubled?
• How could a weak acid change our assumption about neutral pH at equivalence?
• Why can repeated endpoints agree even with small measurement variation?

Measurement model and limits

Each aliquot varies within ±0.03 mL of 20.0 mL; each trial has a fixed delivery factor within ±0.2% of displayed added volume. Probe noise is bounded by ±0.02 pH; display resolution is 0.01 pH. Burette readings are shown to 0.1 mL. The repeated endpoint interval reflects sampling and this explicit variation. It is not a statistical confidence interval.

The model assumes complete dissociation, a 1:1 reaction, additive solution volumes, and water autoionization at 25°C. Activities, indicator volume, atmospheric carbon dioxide, heat transfer, probe lag, and real calibration are omitted. Color is an illustrative universal-indicator palette. This is not a procedure for unsupervised handling of real chemicals.

Background reference: OpenStax acid–base titrations

The student scenario, guided procedure, and assessment criteria are original instructional content.