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

(a) The flowchart below is provided to plan the order of steps for the separation by filtration of the precipitates from the sample of waste water.

Complete the flowchart. For each step state the reagent added and the substance(s) removed by filtration.

step 1
reagent added
HCl(aq)
substance(s) removed by filtration
...............................
...............................

step 2
reagent added
...............................
substance(s) removed by filtration
...............................
...............................

step 3
reagent added
...............................
substance(s) removed by filtration
...............................
...............................
[3]

(b) After the substances formed in each step in (a) have been removed by filtration, a further drop of the reagent used for that step is added to the waste water sample.
Suggest why this is necessary.
...............................
...............................
[1]

(c) Between steps 1 and 2, the waste water sample is neutralised. This is so that steps 2 and 3 will work.
Suggest a reagent that should be added to neutralise the waste water sample. State how you could test that the waste water sample had been neutralised.
reagent ...............................
test ...............................
...............................
...............................
[2]

A student completes the reactions in (a) with a sample of waste water and obtains one of the precipitates produced, XCO₃.

(d) The student plans to calculate the concentration of X²⁺ ions present in this sample of waste water by measuring the mass loss on heating the freshly filtered sample of XCO₃(s).

(i) Draw and label a diagram of the experimental set-up the student should use. Your diagram should show how loss of solid during heating would be prevented.
[2]

The student weighs the XCO₃ precipitate and records its mass. The student then heats the precipitate strongly for five minutes and weighs it again. The student records the difference in mass and from this calculates [X²⁺].

The precipitate thermally decomposes according to the following equation.

$XCO_3(s) \xrightarrow{\text{heat}} XO(s) + CO_2(g)$

(ii) Identify two problems with this method. Explain the effect that each problem has on the calculated concentration of X²⁺.

problem 1 ...............................
effect 1 ...............................
...............................

problem 2 ...............................
effect 2 ...............................
...............................
[4]

The hydrolysis of urea, $(NH_2)_2CO$, $M_r = 60$, is shown by the equation. The reaction is catalysed by the enzyme urease, when present in low concentrations.

$$ (NH_2)_2CO + 2H_2O \rightarrow 2NH_4^+ + CO_3^{2-} $$

The formation of the $NH_4^+$ and $CO_3^{2-}$ ions increases the conductivity of the solution. The increase in conductivity over time can be measured and the rate of hydrolysis can be determined from this.

A student plans to carry out this reaction using a $50 \text{ cm}^3$ sample of $5.00 \times 10^{-3} \text{ mol dm}^{-3}$ urea solution.

(a) The $5.00 \times 10^{-3} \text{ mol dm}^{-3}$ urea solution cannot be accurately prepared by dissolving urea in $50.0 \text{ cm}^3$ of water. A more concentrated solution is first prepared. This is then diluted to make the solution of the required concentration.

(i) Suggest why diluting a more concentrated solution is a more accurate method of preparing this solution than by dissolving urea in $50.0 \text{ cm}^3$ of water.
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.................................................................................................................. [1]
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(ii) Describe how you would accurately prepare exactly $100 \text{ cm}^3$ of $5.00 \times 10^{-3} \text{ mol dm}^{-3}$ urea solution, starting with $3.00 \text{ g}$ of solid urea. Your method should use more than one volumetric flask, including one of capacity $1000 \text{ cm}^3$, and other suitable equipment.
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.................................................................................................................. [3]

(b) A student added a small amount of urease to a $50.0 \text{ cm}^3$ sample of the solution made in (a)(ii). The conductivity of the solution was measured. The student obtained the results shown in the table. The student forgot to record the conductivity at $150 \text{ s}$. Conductivity is measured in $\mu S \text{ cm}^{-1}$.

$$\begin{array}{c|c} \text{time, } t \text{/s} & \text{conductivity } / \mu S \text{ cm}^{-1} \midrule 0 & 0 \30 & 55 \60 & 110 \90 & 160 \120 & 215 \150 & -- \180 & 320 \210 & 375 \240 & 425 \270 & 480 \300 & 530 \end{array}$$

(i) Plot a graph on the grid to show the relationship between time $t$ and conductivity. Use a cross $(\times)$ to plot each data point. Draw the straight line of best fit. [2]

(ii) Use the graph to determine the gradient of the best-fit line. State the co-ordinates of both points you used in your calculation. Give your answer to three significant figures.

co-ordinates 1 ........................................................ co-ordinates 2 .............................................

gradient = ................................. $\mu S \text{ cm}^{-1} \text{ s}^{-1}$ [2]

(iii) Use your graph to determine the conductivity of the solution at $t = 150 \text{ s}$.

conductivity at $150 \text{ s} = ................................. \mu S \text{ cm}^{-1}$ [1]

(c) In another experiment, a student added a small amount of urease to $50.0 \text{ cm}^3$ samples of urea of various concentrations and measured their conductivity until hydrolysis was complete.

(i) Sketch a graph on the axes below to show the variation in conductivity in one such experiment.
Explain the shape of your graph as hydrolysis reaches completion.
................................................................. [2]
[Graph with axes labeled 'conductivity' and 'time']

From their results, the student plotted the calibration graph, shown below, of final conductivity of solution against initial concentration of urea.
[Calibration graph with x-axis labeled 'initial concentration of urea / mol dm$^{-3}$' and y-axis labeled 'final conductivity / $\mu S \text{ cm}^{-1}$']
You are going to determine the average rate of hydrolysis over the first $150 \text{ s}$ of the experiment in (b).

(ii) Use the calibration graph to determine the initial concentration of urea that would give the same final conductivity as your answer to (b)(iii). (If you were unable to obtain an answer to (b)(iii) you should use the value of $350 \mu S \text{ cm}^{-1}$. This is not the correct value.)
initial concentration of urea = .................................... $\text{mol dm}^{-3}$ [1]

(iii) Use your answer to (c)(ii) to calculate the average rate of hydrolysis of urea during the first $150 \text{ s}$. Determine the units of the rate of hydrolysis.
rate of hydrolysis = ............................................... units = .................................... [2]

(d) The student repeats the experiment in (b) using a different low concentration of urea solution.

The student measures the conductivity of the solution for $300 \text{ s}$.

At $300 \text{ s}$, the student adds $1.0 \text{ cm}^3$ of $0.05 \text{ mol dm}^{-3}$ aqueous silver nitrate, $AgNO_3(aq)$. The measured conductivity of the solution immediately increases.

$AgNO_3(aq)$ is known to prevent the enzyme urease from hydrolysing urea.

Using the axes below, sketch a graph to show how the conductivity of the solution changes during this experiment.
Extend the graph beyond the point at which $AgNO_3(aq)$ is added at $300 \text{ s}$.
[Graph with axes labeled 'conductivity' and 'time'] [3]

(e) The following information gives some of the hazards associated with the chemicals used in the procedure.
$\begin{array}{|c|} \hline \text{silver nitrate} & \text{Solutions equal to or more concentrated than 0.18 mol dm}^{-3}\text{ are classified as corrosive and hazardous for the aquatic environment; solutions equal to or more concentrated than 0.06 mol dm}^{-3}\text{ but less concentrated than 0.18 mol dm}^{-3}\text{ are classified as moderate hazard and hazardous for the aquatic environment; solutions less concentrated than 0.06 mol dm}^{-3}\text{ are classified as hazardous for the aquatic environment.} \text{urea} & \text{All solutions are low hazard.} \text{urease} & \text{Concentrated solutions may produce allergic reactions. They can cause asthma and irritate the eyes, nose and skin. Dilute solutions are low hazard.} \hline \end{array}$

Describe one relevant precaution, other than eye protection and a lab coat, that should be taken to keep the risk associated with the chemicals used to a minimum. Explain your answer.
precaution ..................................................................................................................
explanation .................................................................................................................. [1]