Chemistry 9701/53 — May/June 2015

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Analysis, Conclusions and Evaluation · Planning

Q1Analysis, Conclusions and EvaluationPlanningFree sample

A saturated aqueous solution of magnesium methanoate, $\text{Mg(HCOO)}_2$ , has a solubility of approximately $150\text{ g dm}^{-3}$ at room temperature. Its exact solubility can be determined by titrating magnesium methanoate against aqueous potassium manganate(VII).

During the titration, the methanoate ion, $\text{HCOO}^-$ , is oxidised to carbon dioxide while the manganate(VII) ion, $\text{MnO}_4^-$ , is reduced to $\text{Mn}^{2+}$ .

You are supplied with:

a saturated aqueous solution of $\text{Mg(HCOO)}_2$
aqueous potassium manganate(VII), $\text{KMnO}_4$ , of concentration $0.0200\text{ mol dm}^{-3}$

(a)

(i)

Write the half equations for the oxidation of $\text{HCOO}^-\text{(aq)}$ to $\text{CO}_2\text{(g)}$ and the reduction of $\text{MnO}_4^-\text{(aq)}$ to $\text{Mn}^{2+}\text{(aq)}$ in acid solution.

(ii)

Using the approximate solubility above, calculate the concentration, in $\text{mol dm}^{-3}$ , of the saturated aqueous magnesium methanoate and the concentration of the methanoate ions present in this solution.

[ $A_r$ : H, 1.0; C, 12.0; O, 16.0; Mg, 24.3]

(iii)

In order to obtain a reliable titre value, the saturated solution of magnesium methanoate needs to be diluted.

Describe how you would accurately measure a $5.0\text{ cm}^3$ sample of saturated magnesium methanoate solution and use it to prepare a solution fifty times more dilute than the saturated solution.

(iv)

Before the titration is carried out, dilute sulfuric acid must be added to the magnesium methanoate.

Explain why this is necessary and also whether the volume of sulfuric acid chosen will affect the result of the titration.

(v)

The potassium manganate(VII) is added from a burette into the magnesium methanoate in a conical flask.

Describe what you would see when you had reached the end-point of the titration.

(vi)

$1\text{ mol}$ of acidified $\text{MnO}_4^-$ ions reacts with $2.5\text{ mol}$ of $\text{HCOO}^-$ ions.

$25.0\text{ cm}^3$ of the diluted solution prepared in (iii) required $25.50\text{ cm}^3$ of $0.0200\text{ mol dm}^{-3}$ potassium manganate(VII) solution to reach the end-point.

Use this information to calculate the concentration, in $\text{mol dm}^{-3}$ , of $\text{HCOO}^-$ ions in the diluted solution.

(vii)

Use your answer to (vi) to calculate the concentration, in $\text{mol dm}^{-3}$ , of the saturated solution of magnesium methanoate, $\text{Mg(HCOO)}_2$ . Give your answer to three significant figures.

(b)

The solubility of magnesium methanoate can be determined at higher temperatures using the same titration.

In an experiment to determine how the concentration of saturated magnesium methanoate varies with temperature, name the independent variable and the dependent variable.

independent variable

dependent variable

(c)

The solubility of magnesium methanoate increases with temperature.

What does this tell you about $\Delta H$ for the process below?

\text{Mg(HCOO)}_2\text{(s)} \rightleftharpoons \text{Mg}^{2+}\text{(aq)} + 2\text{HCOO}^-\text{(aq)}

Explain your answer.

(d)

A student used the same titration method, this time to measure the concentration of a saturated solution of barium methanoate.

Explain why the acidification of the solution with dilute sulfuric acid might make the titration difficult to do.

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