Why must the acid used in a back titration be present in excess?

Only an excess guarantees that all of the solid has reacted, so the difference between the acid added and the acid remaining measures the whole sample.

Why is the reacted mixture made up to a known volume and titrated in aliquots?

One sample then provides several identical portions, so repeat titrations can be done and a mean concordant titre obtained without preparing the sample again.

A 2.00 g sample of limestone is added to 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid and the mixture made up to 250 cm³. A 25.0 cm³ aliquot needs 14.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide. Calculate the percentage of calcium carbonate.

The aliquot contains 0.100 × 0.0140 = 1.40 × 10⁻³ mol of unreacted acid, so the 250 cm³ holds 0.0140 mol. Acid used by the carbonate is 0.0500 − 0.0140 = 0.0360 mol, giving 0.0180 mol of CaCO₃ weighing 1.80 g, which is 90.1% of the sample.

Which indicator is suitable for the final stage of a limestone back titration, and what colour change is seen?

Phenolphthalein is suitable because the remaining acid and the sodium hydroxide are both strong, and the end point is the first permanent change from colourless to pale pink.

Eduqas A-Level Chemistry A410QS

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Specified Practical Work

Back titration and double titration

3 things to know
Component 3

Overview

Recall questions on when a back titration is used in preference to a direct one, the method and the calculation by difference, why the reagent must be in genuine excess, the double titration with two indicators, and worked purity determinations.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What effect does a sodium hydroxide solution having stood open to the air have on the result of a double titration?

Key term

Why is the percentage of calcium carbonate in limestone found by a back titration rather than by titrating the solid directly?

Process

Outline the procedure for determining the percentage of calcium carbonate in limestone by back titration.

Questions from this subtopic

Each one with the model answer underneath.

1
Why is the percentage of calcium carbonate in limestone found by a back titration rather than by titrating the solid directly?
Model answer
Limestone is an insoluble solid that reacts slowly and cannot be delivered from a burette, so there is no way to judge an end point against it. Dissolving it in a known excess of acid and measuring how much acid is left avoids the problem.
2
Outline the procedure for determining the percentage of calcium carbonate in limestone by back titration.
Model answer
A weighed sample of powdered limestone is added to a measured volume of standard hydrochloric acid in excess and warmed until effervescence stops. The solution is made up to a known volume in a volumetric flask, and aliquots are titrated against standard sodium hydroxide to find the acid remaining.
3
Why must the acid used in a back titration be present in excess?
Model answer
Only an excess guarantees that all of the solid has reacted, so the difference between the acid added and the acid remaining measures the whole sample.
4
Why is the reacted mixture made up to a known volume and titrated in aliquots?
Model answer
One sample then provides several identical portions, so repeat titrations can be done and a mean concordant titre obtained without preparing the sample again.
5
A 2.00 g sample of limestone is added to 50.0 cm³ of 1.00 mol dm⁻³ hydrochloric acid and the mixture made up to 250 cm³. A 25.0 cm³ aliquot needs 14.0 cm³ of 0.100 mol dm⁻³ sodium hydroxide. Calculate the percentage of calcium carbonate.
Model answer
The aliquot contains 0.100 × 0.0140 = 1.40 × 10⁻³ mol of unreacted acid, so the 250 cm³ holds 0.0140 mol. Acid used by the carbonate is 0.0500 − 0.0140 = 0.0360 mol, giving 0.0180 mol of CaCO₃ weighing 1.80 g, which is 90.1% of the sample.
6
Which indicator is suitable for the final stage of a limestone back titration, and what colour change is seen?
Model answer
Phenolphthalein is suitable because the remaining acid and the sodium hydroxide are both strong, and the end point is the first permanent change from colourless to pale pink.

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