Chemistry 9701/31 — May/June 2011

Background Concept

Titration is a quantitative volumetric technique for determining the exact concentration of a solution by reacting a measured volume of it with a solution of known concentration (a standard solution). Here, sulfuric acid (FA 1, approximately 0.7 mol dm⁻³) is first diluted tenfold: 25.0 cm³ is pipetted into a 250 cm³ volumetric flask and made up to the mark with distilled water, giving FA 3 (approximately 0.07 mol dm⁻³). The diluted acid is then titrated against sodium hydroxide (FA 2, 0.150 mol dm⁻³) using phenolphthalein indicator. The endpoint is signalled by a permanent pale pink colour, which appears as soon as the sodium hydroxide is in very slight excess (the solution becomes alkaline).

The key practical skills being assessed are:

• Using a burette correctly and reading it to the nearest 0.05 cm³.
• Performing a rough titration to find the approximate endpoint quickly.
• Repeating accurate titrations until at least two concordant results (within 0.1 cm³ of each other) are obtained.
• Recording results clearly in a table with appropriate headings and units.

Understanding the Question

This part asks you to record the rough titre and then carry out accurate titrations, tabulating all burette readings and the volume of FA 2 added. The 7 marks reward the layout of the results (a clear table), the completeness of the recordings (initial and final readings plus titre for every accurate run), the precision of the readings (to 0.05 cm³), and the quality of the results (concordant titres within 0.1 cm³, and close agreement with the supervisor's value).

Approach

1. Perform a rough titration first, adding FA 2 quickly until the colour changes, to find the approximate titre.
2. Then carry out accurate titrations: record the initial burette reading, add FA 2 slowly with swirling until the pale pink colour persists, and record the final reading.
3. Calculate each titre as final − initial.
4. Repeat until at least two concordant titres (within 0.1 cm³) are obtained.
5. Present all results in a clear table with headings and units.

Step-by-Step Reasoning

The rough titration is a quick first estimate. You run FA 2 into the acid in larger portions until the colour changes, giving an approximate titre (here 23.50 cm³). This tells you roughly where the endpoint is, so the accurate titrations can be done efficiently — you can add most of the alkali quickly and slow down near the endpoint.

For each accurate titration:

• Record the initial burette reading (e.g. 0.00 cm³).
• Add FA 2 from the burette, swirling the conical flask continuously, until the pale pink colour just persists.
• Record the final burette reading (e.g. 23.35 cm³).
• The titre is the difference: 23.35 − 0.00 = 23.35 cm³.

A burette graduated in 0.1 cm³ divisions can be read to the nearest 0.05 cm³ (half a division). All readings must be recorded to this precision.

Concordant titres are those within 0.1 cm³ of each other. In the example, titres of 23.35, 23.35 and 23.30 cm³ are concordant (the largest difference is 0.05 cm³). You should keep titrating until you have at least two concordant results.

Example table:

Key Takeaways

• Always do a rough titration before accurate ones.
• Read the burette to the nearest 0.05 cm³.
• Aim for at least two concordant titres within 0.1 cm³.
• Record results in a clear, labelled table.

Common Mistakes

• Recording readings to only 1 decimal place (0.1 cm³) — the mark scheme requires 0.05 cm³ precision.
• Omitting initial readings or the titre volume.
• Using 50.00 cm³ as an initial burette reading — the mark scheme explicitly disallows this.
• Stopping after one accurate titration without checking concordance.

Things to Be Careful About

• Headings in the table must match the readings (e.g. "Volume of FA 2 added / cm³").
• No burette reading may exceed 50.00 cm³.
• The indicator should remain colourless before the titration begins; if it turns pink, the flask has been contaminated.
• The endpoint is a permanent pale pink — a transient pink that disappears on swirling is not the endpoint.

Background Concept

A single titration reading is subject to random error, so the most reliable estimate of the true titre is the mean of several concordant readings. "Concordant" here means within 0.20 cm³ of each other (the mark scheme for part (b) uses 0.20 cm³, while part (a) uses 0.1 cm³ for the concordance quality mark).

Understanding the Question

From the accurate titrations in part (a), you must select a suitable value — the mean of the concordant titres — to use in the calculations. You must show how you obtained it, either as an expression or by ticking the titres used in the table.

Approach

1. Identify the concordant titres (within 0.20 cm³ of each other).
2. Calculate their mean.
3. Give the mean to 2 decimal places (or to 1 decimal place if all readings were to 1 dp and the mean is exactly correct).

Step-by-Step Reasoning

With titres 23.35, 23.35 and 23.30 cm³, all three are within 0.20 cm³ of each other, so all are used:

$$\text{mean} = \frac{23.35 + 23.35 + 23.30}{3} = 23.33 \text{ cm}^3$$

This mean is the value used in all subsequent calculations. The mark scheme allows the mean to be rounded to the nearest 0.05 cm³ if desired, but 23.33 cm³ is acceptable.

Key Takeaways

• The mean of concordant titres is the best estimate of the true titre.
• Show which titres you averaged, clearly.

Common Mistakes

• Averaging titres that are not concordant (more than 0.20 cm³ apart).
• Using only one titre without justification.
• Not showing the mean calculation.

Things to Be Careful About

• The mark scheme requires the titres used to be clearly shown (in an expression or ticked in the table).
• If all readings are to 1 decimal place, the mean may be given to 1 decimal place only if it is numerically correct without rounding.