Chemistry 9701/52 — May/June 2018

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

Q1PlanningAnalysis, Conclusions and EvaluationFree sample

When concentrated iron(III) chloride is added to water at just below boiling point, a reaction occurs and produces $\text{Fe}_2\text{O}_3$ , seen as a red colour in the water. This is a ‘sol’ of $\text{Fe}_2\text{O}_3$ . A sol contains particles that are insoluble but do not form a precipitate.

A student prepared a concentrated solution of iron(III) chloride by dissolving $\text{FeCl}_3\cdot6\text{H}_2\text{O(s)}$ in distilled water.

(a)

Hazard information for hydrated iron(III) chloride is given.

For this hazard, state a precaution, other than eye protection and a lab coat, that the student could take when preparing a solution of concentrated iron(III) chloride.

hazard: solid $\text{FeCl}_3\cdot6\text{H}_2\text{O}$ is irritating to the skin

precaution

(b)

Particles of a sol can be positively or negatively charged. The student used the experimental set-up shown to confirm that the $\text{Fe}_2\text{O}_3$ sol particle is positively charged.

Fig. 1.1

The student placed a few $\text{cm}^3$ of the sol at the bottom of the U-tube and poured $10\text{ cm}^3$ of distilled water into each side of the U-tube, without disturbing the sol. The two layers of distilled water were colourless at the beginning of the experiment. Graphite electrodes were inserted and a current was passed. After 30 minutes a difference was noted between the distilled water in the two sides of the U-tube.

Predict the colour of the distilled water in both sides of the U-tube after 30 minutes, if the $\text{Fe}_2\text{O}_3$ sol particle is positively charged.

observation in side with positive electrode

observation in side with negative electrode

(c)

Salt solutions can be added to sols to cause them to precipitate. This method is used in water purification.

The student made up $100.0\text{ cm}^3$ of standard solutions containing $0.100\text{ mol dm}^{-3}$ of the following ions.

\text{K}^\text{+}(\text{aq}) \quad \text{Mg}^{2+}(\text{aq}) \quad \text{Al}^{3+}(\text{aq}) \quad \text{Cl}^-(\text{aq}) \quad \text{SO}_4^{2-}(\text{aq}) \quad \text{PO}_4^{3-}(\text{aq})

(i)

What mass of solid potassium sulfate, $\text{K}_2\text{SO}_4$ , did the student use to make up exactly $100.0\text{ cm}^3$ of $0.100\text{ mol dm}^{-3}$ $\text{SO}_4^{2-}(\text{aq})$ ?
[ $A_r$ : K, 39.1; S, 32.1; O, 16.0]

mass of $\text{K}_2\text{SO}_4$ =

(ii)

Describe how the student should have accurately prepared this volume of standard solution from a sample of $\text{K}_2\text{SO}_4$ of mass calculated in (c)(i).

(d)

The student carried out an experiment to precipitate the $\text{Fe}_2\text{O}_3$ sol, using $0.100\text{ mol dm}^{-3}$ $\text{K}_2\text{SO}_4(\text{aq})$ . Only one drop of $\text{K}_2\text{SO}_4(\text{aq})$ was needed for the complete precipitation of $10.0\text{ cm}^3$ $\text{Fe}_2\text{O}_3$ sol.

Calculate how many moles of $\text{SO}_4^{2-}$ were added. Assume that one drop is $0.05\text{ cm}^3$ .

moles of $\text{SO}_4^{2-}$ added =

(e)

The student decided to dilute the standard solution of $0.100\text{ mol dm}^{-3}$ $\text{K}_2\text{SO}_4$ to make $50.0\text{ cm}^3$ of $0.0100\text{ mol dm}^{-3}$ $\text{K}_2\text{SO}_4(\text{aq})$ .

(i)

Calculate the volume of standard solution required to make exactly $50.0\text{ cm}^3$ of $0.0100\text{ mol dm}^{-3}$ $\text{K}_2\text{SO}_4(\text{aq})$ .

volume of standard $\text{K}_2\text{SO}_4(\text{aq})$ =

(ii)

Name a piece of apparatus that could be used to measure accurately the volume of solution calculated in (e)(i).

(f)

In an alternative method, $50.0\text{ cm}^3$ of $0.0100\text{ mol dm}^{-3}$ $\text{K}_2\text{SO}_4(\text{aq})$ could be prepared by using $0.0872\text{ g}$ of $\text{K}_2\text{SO}_4$ .

Explain why the dilution method used by the student to prepare $50.0\text{ cm}^3$ of $0.0100\text{ mol dm}^{-3}$ $\text{K}_2\text{SO}_4(\text{aq})$ is the more accurate of the two methods.

(g)

The student carried out experiments to investigate how much of a particular salt solution was required to fully precipitate all the $\text{Fe}_2\text{O}_3$ sol in a $1000\text{ cm}^3$ sample. The salt solutions used were all of concentration $0.0100\text{ mol dm}^{-3}$ with respect to the ion being investigated.

Experiment 1

identity of salt solution	charge on anion	minimum amount of anion required for complete precipitation of $1000\text{ cm}^3$ sol/mol
$\text{KCl}$	-1	$1.02 \times 10^{-1}$
$\text{K}_2\text{SO}_4$	-2	$3.25 \times 10^{-4}$
$\text{K}_3\text{PO}_4$	-3	$8.56 \times 10^{-5}$

Experiment 2

identity of salt solution	charge on cation	minimum amount of cation required for complete precipitation of $1000\text{ cm}^3$ sol/mol
$\text{KCl}$	+1	$1.02 \times 10^{-1}$
$\text{MgCl}_2$	+2	$1.10 \times 10^{-1}$
$\text{AlCl}_3$	+3	$1.15 \times 10^{-1}$

(i)

Describe the effect of changing the charge on the anion from –1 to –2 to –3 on the precipitation of the $\text{Fe}_2\text{O}_3$ sol in Experiment 1.

(ii)

Identify the independent variable in Experiment 2.

(iii)

Arsenic sulfide, $\text{As}_2\text{S}_3$ , is highly toxic and should be removed from drinking water.

The $\text{Fe}_2\text{O}_3$ sol particles are positively charged.

The $\text{As}_2\text{S}_3$ sol particles are negatively charged.

Based on the student’s results, which salt used in either Experiment 1 or Experiment 2 would be the most effective at removing $\text{As}_2\text{S}_3$ from drinking water?
Explain your answer.

salt

explanation

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