AS & A Level Chemistry - 9701 Paper 5 2018 Summer Zone 2

When concentrated iron(III) chloride is added to water at just below boiling point, a reaction occurs and produces $Fe_{2}O_{3}$, seen as a red colour in the water. This is a 'sol' of $Fe_{2}O_{3}$. 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 $FeCl_{3}.6H_{2}O$(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 $FeCl_{3}.6H_{2}O$ is irritating to the skin
precaution ...................................................
[1]

Particles of a sol can be positively or negatively charged. The student used the experimental set-up shown to confirm that the $Fe_{2}O_{3}$ sol particle is positively charged.



(b) The student placed a few cm³ of the sol at the bottom of the U-tube and poured 10 cm³ 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 $Fe_{2}O_{3}$ sol particle is positively charged.
observation in side with positive electrode ...................................................
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observation in side with negative electrode ...................................................
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Salt solutions can be added to sols to cause them to precipitate. This method is used in water purification.

(c) The student made up 100.0 cm³ of standard solutions containing $0.100$ mol dm^-3 of the following ions.
$K^{+}$(aq) $Mg^{2+}$(aq) $Al^{3+}$(aq) $Cl^{-}$(aq) $SO_{4}^{2-}$(aq) $PO_{4}^{3-}$(aq)

(i) What mass of solid potassium sulfate, $K_{2}SO_{4}$, did the student use to make up exactly 100.0 cm³ of $0.100$ mol dm^-3 $SO_{4}^{2-}$(aq)?
[A_r: K, 39.1; S, 32.1; O, 16.0]

mass of $K_{2}SO_{4}$ = .............................. g [1]

(ii) Describe how the student should have accurately prepared this volume of standard solution from a sample of $K_{2}SO_{4}$ of mass calculated in (c)(i).
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(d) The student carried out an experiment to precipitate the $Fe_{2}O_{3}$ sol, using $0.100$ mol dm^-3 $K_{2}SO_{4}$(aq). Only one drop of $K_{2}SO_{4}$(aq) was needed for the complete precipitation of 10.0 cm³ $Fe_{2}O_{3}$ sol.

Calculate how many moles of $SO_{4}^{2-}$ were added. Assume that one drop is 0.05 cm³.
moles of $SO_{4}^{2-}$ added = ....................... mol [1]

(e) The student decided to dilute the standard solution of $0.100$ mol dm^-3 $K_{2}SO_{4}$ to make 50.0 cm³ of $0.0100$ mol dm^-3 $K_{2}SO_{4}$(aq).

(i) Calculate the volume of standard solution required to make exactly 50.0 cm³ of $0.0100$ mol dm^-3 $K_{2}SO_{4}$(aq).

volume of standard $K_{2}SO_{4}$(aq) = .............................. cm³ [1]

(ii) Name a piece of apparatus that could be used to measure accurately the volume of solution calculated in (e)(i).
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(f) In an alternative method, 50.0 cm³ of $0.0100$ mol dm^-3 $K_{2}SO_{4}$(aq) could be prepared by using 0.0872 g of $K_{2}SO_{4}$.

Explain why the dilution method used by the student to prepare 50.0 cm³ of $0.0100$ mol dm^-3 $K_{2}SO_{4}$(aq) is the more accurate of the two methods.
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(g) The student carried out experiments to investigate how much of a particular salt solution was required to fully precipitate all the $Fe_{2}O_{3}$ sol in a 1000 cm³ sample. The salt solutions used were all of concentration $0.0100$ mol dm^-3 with respect to the ion being investigated.

$$\begin{array}{|c|c|c|} \hline \text{identity of salt solution} & \text{charge on anion} & \text{minimum amount of anion required} \\ \hline KCl & -1 & 1.02 \times 10^{-1} \\ \hline K_{2}SO_{4} & -2 & 3.25 \times 10^{-4} \\ \hline K_{3}PO_{4} & -3 & 8.56 \times 10^{-5} \\ \hline \end{array}$$

Experiment 2
$$\begin{array}{|c|c|c|} \hline \text{identity of salt solution} & \text{charge on cation} & \text{minimum amount of cation required} \\ \hline KCl & +1 & 1.02 \times 10^{-1} \\ \hline MgCl_{2} & +2 & 1.10 \times 10^{-1} \\ \hline AlCl_{3} & +3 & 1.15 \times 10^{-1} \\ \hline \end{array}$$

(i) Describe the effect of changing the charge on the anion from $-1$ to $-2$ to $-3$ on the precipitation of the $Fe_{2}O_{3}$ sol in Experiment 1.
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(ii) Identify the independent variable in Experiment 2.
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(iii) Arsenic sulfide, $As_{2}S_{3}$, is highly toxic and should be removed from drinking water.
The $Fe_{2}O_{3}$ sol particles are positively charged.
The $As_{2}S_{3}$ 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 $As_{2}S_{3}$ from drinking water? Explain your answer.
salt ....................................................................................
explanation ....................................................................................
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Water boils when the pressure of its vapour above the liquid surface is equal to the atmospheric pressure. When substances are dissolved in water, the vapour pressure of the water is reduced and its boiling point is increased.
The increase in boiling point is known as the boiling point elevation, $\Delta T$, which is the difference between the boiling point of a solution and the boiling point of pure water. $\Delta T$ is usually small, often less than 1 $\degree$C.
When glucose is dissolved in 1 kg of water, the relationship between $\Delta T$ and the number of moles of glucose dissolved is as shown.
$$\Delta T = K_b \times Z$$
$K_b$ is the boiling point constant of pure water
$$Z = \frac{\text{number of moles of glucose}}{\text{mass of water, in kg}}\text{ in mol kg}^{-1}$$
(a) Use the information above to explain why lowering the vapour pressure of a liquid increases the temperature at which it boils.

(b) Show, using a labelled arrow, where the cooling water enters the reflux condenser. [1]
(c) A digital probe thermometer is used as shown in the diagram.
Explain why a normal laboratory glass thermometer would not be suitable.

The student follows this procedure.
1 Transfer 75.00 g of distilled water to the round-bottomed flask.
2 Add anti-bumping granules to the distilled water to prevent violent, uneven boiling.
3 Heat the distilled water until it boils and record the highest stable temperature.
4 Stop heating and allow the distilled water to cool to room temperature.
5 Remove the reflux condenser and add about 1 g of anhydrous glucose, measured accurately.
6 Replace the reflux condenser and heat the solution until it boils, noting the highest stable temperature.
7 Repeat steps 4 to 6, each time adding approximately 1 g more of anhydrous glucose, accurately weighed, until sufficient readings are taken.
(d) In step 4, the heating is stopped and the distilled water allowed to cool from its boiling point, before removing the reflux condenser.
Apart from for safety reasons, explain why this is essential.

(e) At 101 kPa (1 atm), distilled water is known to boil at 100.00 $\degree$C.
Suggest why the boiling point of distilled water in this experiment was found to be 99.48 $\degree$C.
Assume that the digital probe thermometer was reading correctly.

(f) (i) The student constructed the table shown to record the results for this experiment.
Complete columns C and D to three significant figures and column E to two decimal places.
[The $M_r$ of glucose is 180.]
$Z = \frac{\text{moles of glucose}}{\text{mass of water, in kg}}$