CAIE A Level 9701

Chemistry 2020 · May/June

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18 Questions Q&A Available
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This CAIE A Level Chemistry paper focuses mainly on quantitative chemistry, gas laws, experimental design, and redox/transition-metal chemistry, with a strong emphasis on chlorides of iron and practical analysis. Students can expect structured questions that combine calculations, graph plotting, data interpretation, and short explanatory responses, rather than simple recall; many parts require using \(PV=nRT\), mole relationships, percentage error, gradients, and stoichiometric reasoning.…

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Questions & Model Answers 18 Qs

Q1a(i)
Short Answer 2 marks
You should assume that gas X behaves like an ideal gas and so use PV = nRT. R = 8.31 J K^-1 mol^-1. Calculate the moles of gas X.
Use n = PV / RT with the given pressure, volume and temperature values from Experiment 1 to calculate the amount of gas X in mol.
The ideal gas equation is rearranged to find n.
Q1a(ii)
Short Answer 1 mark
The sample of gas X is found to have a mass of 0.251 g. Explain how the student should use this information and their results to determine the Mr of X.
Divide the mass of gas X by the number of moles calculated in part (i) to obtain Mr.
Mr = mass / moles.
Q1a(iii)
Short Answer 1 mark
Not all of gas X is absorbed by the soda lime. State what effect, if any, this has on the student's calculated value of the Mr of gas X.
The calculated Mr will be too low.
If some gas is not absorbed, the measured mass loss is too small, so the calculated number of moles is too low and Mr is underestimated.
Q1a(iv)
Short Answer 1 mark
In Experiment 2, the same mass of gas X is used, but the student did not record the temperature and pressure. The calculated Mr of X for Experiment 2 is higher than the value calculated by the student for Experiment 1. State and explain how the value of P/T changes from Experiment 1 to Experiment 2.
P/T decreases from Experiment 1 to Experiment 2.
For the same mass of gas, a higher calculated Mr means fewer moles were calculated from PV = nRT, so P/T must be smaller.
Q1e
Short Answer 1 mark
State how the reliability of the results in Experiment 1 could be improved.
Repeat the experiment and calculate a mean result, ignoring anomalous results.
Repeating measurements improves reliability.
Q1f
Short Answer 1 mark
A different gas, methylamine, is alkaline. State a change that would have to be made to the apparatus so that the Mr of methylamine could be determined.
Replace the soda lime with an acidic absorbent, such as dilute hydrochloric acid, so that methylamine is absorbed.
An alkaline gas will not be absorbed by soda lime, so the absorbent must be changed.
Q2a(i)
Short Answer 1 mark
Each student carries out the following steps in the order shown in the list. Some of the measurement steps are missing from the list. State the two additional measurement steps that each student must perform in order to find the formula of the chloride of iron.
1. Weigh the reaction tube containing the iron wire after reaction. 2. Weigh the side-arm conical flask before reaction / weigh the empty flask.
Masses before and after reaction are needed to determine the mass of iron chloride formed and the mass of chlorine gained.
Q2a(ii)
Short Answer 1 mark
The flow of dry chlorine gas must start before the iron wire is heated. Explain why.
To exclude air/oxygen and prevent the iron from reacting with oxygen or oxidising before chlorine is present.
Chlorine must be present first so the iron reacts with chlorine rather than air.
Q2a(iii)
Short Answer 1 mark
State an assumption that has to be made for the measurements made in this experiment to be valid.
All of the iron reacts completely and all of the chloride of iron produced is collected/weighed.
The calculation assumes no product is lost and the reaction goes to completion.
Q2b(i)
Structured 3 marks
The class results are shown in the table. Calculate the mass of chlorine that reacts with the iron wire in each experiment. Record each mass to two decimal places. Calculate the amount of iron, in mol, and amount of chlorine atoms, in mol, that reacts in each experiment. Record the number of moles to three significant figures. [Ar: Fe, 55.8; Cl, 35.5]
For each student, mass of chlorine = mass of iron chloride - mass of iron. Amount of iron = mass of iron / 55.8. Amount of chlorine atoms = mass of chlorine / 35.5. The completed values are to be calculated from the table.
These are the required calculations for the table.
Q2b(ii)
Structured 2 marks
Plot a graph on the grid of the amount of chlorine atoms against the amount of iron. Use a cross (×) to plot each data point. Draw a line of best fit through the plotted points. You should consider whether (0,0) should be on the line of best fit.
Plot the data points and draw a straight line of best fit, considering whether it should pass through the origin.
The graph is a practical/graphical response.
Q2b(iii)
Short Answer 2 marks
Circle the point on the graph you consider to be most anomalous. Suggest one reason why this anomaly may have occurred during this experimental procedure.
The anomalous point is the one furthest from the line of best fit. A possible reason is that some product was lost / the reaction was incomplete / measurement error occurred.
An anomalous point is identified by its deviation from the trend.
Q2c
Structured 2 marks
Use the graph to determine the gradient of the line of best fit. State the coordinates of both points you used in your calculation. These must be on your line of best fit. Give your answer to three significant figures.
Gradient = rise / run using two points on the best-fit line; the exact value depends on the plotted graph.
The gradient must be read from the student's line of best fit.
Q2d
Short Answer 1 mark
The formula of the chloride of iron produced in this experiment is FeCl3. State how the results student 4 obtains could be used to determine this formula.
Use the ratio of moles of iron to moles of chlorine atoms from student 4's results to obtain the simplest whole-number ratio Fe : Cl = 1 : 3.
The empirical formula is found from the mole ratio.
Q2e(i)
Short Answer 1 mark
Student 3 weighs the conical flask using a balance accurate to two decimal places and records its mass. After the chloride of iron is produced the mass increases by 4.03 g. Calculate the percentage error in measuring the mass of this chloride of iron.
Percentage error = (0.01 / 4.03) × 100 = 0.248% (approximately 0.25%).
A balance accurate to two decimal places has an uncertainty of ±0.01 g in the measured mass increase.
Q2e(ii)
Short Answer 1 mark
Student 8 follows the same procedure as student 3. State whether the results from student 8 will have more or less percentage error than those from student 3. Explain your answer.
Less percentage error, because a larger mass gives a smaller percentage uncertainty for the same balance uncertainty.
Percentage error decreases as the measured quantity increases.
Q2f(i)
Short Answer 1 mark
A sample of a chloride of iron prepared by reaction between iron wire and hydrochloric acid contains 44% iron by mass. Show that the formula of this chloride of iron is FeCl2. [Ar: Fe, 55.8; Cl, 35.5]
For FeCl2, percentage iron by mass = 55.8 / (55.8 + 2 × 35.5) × 100 = 44.0%, so the formula is FeCl2.
The calculated percentage matches the given value.
Q2f(ii)
Short Answer 1 mark
Explain, using the electrode potential values in the table, why the methods in (a) and (f) do not produce the same chlorides of iron. [Fe2+(aq) + 2e- -> Fe(s), Eo = -0.44 V; 2H+(aq) + 2e- -> H2(g), Eo = 0.00 V; Fe3+(aq) + e- -> Fe2+(aq), Eo = +0.77 V]
Chlorine is a stronger oxidising agent than H+ because Fe2+/Fe is more negative than H+/H2, so chlorine can oxidise Fe2+ to Fe3+, whereas hydrochloric acid cannot.
The different oxidising strengths lead to different oxidation states of iron.
Paper Details
Board CAIE
Level A Level
Subject Chemistry
Code 9701
Year 2020
Session May/June
Q&A 18 questions
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