A-LevelChemistryChemical energetics9701/33 · May/June 2021 · Q210 Marks

2 In this experiment you will determine the value of x in the formula for hydrated sodium thiosulfate, Na₂S₂O₃•xH₂O. In this formula x is an integer. FA 6 is hydrated sodium thiosulfate, Na₂S₂O₃•xH₂O.

(a)Method • Support the cup in the 250 cm³ beaker. • Rinse the measuring cylinder thoroughly with distilled water. • Use the measuring cylinder to transfer 20.0 cm³ of distilled water into the cup. • Measure and record the initial temperature of the water in the cup. Tilt the cup if necessary so that the bulb of the thermometer is fully covered. • Weigh the stoppered container of FA 6 and record the mass. • Carefully add all the sample of FA 6 to the water in the cup. • Stir the mixture and record the minimum temperature. • Reweigh the stoppered container and any residual FA 6. Record the mass. • Calculate and record the mass of FA 6 added to the water and the change in temperature. I II III IV[4]
(b)(i)Calculate the heat energy change in the reaction. (Assume that 4.2 J are required to change the temperature of 1.0 cm³ of solution by 1.0 °C.) heat energy change = J[1]
(b)(ii)Use your answer to (b)(i) to calculate the relative formula mass, Mr, of hydrated sodium thiosulfate. You should assume that the enthalpy change of hydration, ∆Hhyd, for Na₂S₂O₃•xH₂O is +47.4 kJ mol⁻¹ under the conditions of your experiment. Show your working. Mr of Na₂S₂O₃•xH₂O = [2]
(b)(iii)Use your answer to (b)(ii) to calculate the value of x in the formula Na₂S₂O₃•xH₂O. x = [1]
(c)A student repeats the experiment but adds 25.0 cm³ of distilled water into the cup instead of 20.0 cm³. The student carries out the calculations based on adding 20.0 cm³. What effect would this have on the student's answer to (b)(ii)? Explain your answer.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] I Table / list to include initial and final balance reading, initial and final thermometer reading, mass added and temperature change. and all entries with correct units: / °C, (g) or in °C or in degrees Celcius, in g or in gram(me)s and recorded in the space provided
[1] II All balance readings recorded to the same precision (minimum 1 dp) and all thermometer readings recorded to .0 or .5 °C.
[1] III Accuracy marks: Award if δ ≤ 0.40 °C g⁻¹
[2] IV Accuracy marks: Award if δ < 0.20 °C g⁻¹ (2)
[1] Correctly calculates heat energy change = 20 × 4.2 × temperature change and answer given to 2–4 sf
[2] M1: Correct display of mass of FA 6 / moles of thiosulfate or (b)(i)/47400 M2: Correct use of (correct mass FA 6 × 47400) / ans (b)(i) and answer given to 2–4 sf
[1] Correct display x = [(b)(ii) – 158.2]/ 18 and answer given to the nearest integer
[2] M1: temperature fall / change would be less OR heat energy change would be less M2: therefore moles/ amount of thiosulfate would be fewer/less and so Mr would be greater