A-LevelChemistryAnalytical techniques9701/31 · May/June 2022 · Q211 Marks

In this experiment you will identify a magnesium compound by thermal decomposition. When heated this compound decomposes to give magnesium oxide. FA 4 is the magnesium compound.

(a)Method • Weigh the empty crucible with its lid. Record the mass. • Transfer all the FA 4 from the container into the crucible. • Weigh the crucible, lid and FA 4. Record the mass. • Calculate the mass of FA 4. Record the mass. • Place the crucible and contents on a pipe-clay triangle. • Heat the crucible gently, without the lid, for approximately 2 minutes. • Heat strongly for a further 4 minutes. • Place the lid on the crucible and leave it to cool for at least 5 minutes. During the cooling period, you may wish to begin work on Question 3. • When the crucible is cool, weigh the crucible with its lid and contents. Record the mass. • Heat strongly, without the lid, for a further 2 minutes. • Replace the lid and leave the crucible to cool for at least 5 minutes. • When the crucible has cooled, reweigh the crucible with its lid and contents. Record the mass. • Calculate the mass of residue obtained. Record the mass. Results[5]
(b)(i)Calculations Calculate the amount, in mol, of magnesium oxide produced in your experiment. amount of MgO = ..................................................... mol[1]
(b)(ii)1 mole of FA 4 decomposes on heating to produce 1 mole of MgO and 1 mole of gas X. Calculate the relative formula mass, Mᵣ, of X. Mᵣ of X = .....................................................[1]
(b)(iii)X contains one or more oxygen atoms. Suggest the identity of X. X is .....................................................[1]
(b)(iv)Deduce the name of FA 4. FA 4 is .....................................................[1]
(c)A student suggests that this experiment will be more accurate if FA 4 is heated throughout the experiment with a lid on the crucible. State whether the student is correct. Explain your answer.[1]
(d)State the uncertainty in a single reading of your balance. uncertainty = ± ..................................................... g Calculate the maximum percentage error in the mass of residue that you obtained. Show your working. maximum percentage error = ..................................................... %[1]

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[1] I Unambiguous headings and units for four weighings: • (mass of) crucible, lid (empty) • (mass of) crucible, lid and FA 4 (or 'contents before heating') • (mass of) crucible, lid and MgO / residue after first heating • (mass of) crucible, lid and MgO / residue after second heating
[1] II Readings are appropriately recorded: • all weighings recorded to same decimal places (two or more) • mass of FA 4 is within range 0.80 g-1.80 g (from weighings) • fourth weighing within +0.02 and -0.05 g of third weighing
[1] III Correct subtractions to give masses of FA 4 and MgO / residue
[2] IV and V: Accuracy marks Calculate supervisor's mass ratio (to 2 d.p.) = mass FA 4 / mass of residue Calculate the candidate's mass ratio (to 2 d.p.) = mass FA 4 / mass of residue Calculate 20% of this ratio and calculate ratio ± this ratio Award IV if δ is within the range 0.00-0.25 Award V if δ is within the range 0.00-0.10
[1] Correctly calculates moles of MgO = mass of residue / 40.3 AND answer to 2-4 sig fig
[1] Correctly uses (b)(i) to calculate Mr of X = mass lost / moles of MgO OR Mr of X = (mass FA 4 / moles of MgO) – 40.3 AND answer to 2-4 sf
[1] X is water/steam / H₂O / CO₂
[1] FA 4 is magnesium hydroxide
[1] Student is not correct because there is no spitting / frothing during heating OR student is correct because there was spitting / frothing during heating
[1] 2 d.p. balance uncertainty = 0.01 g or 0.005 g 3 d.p. balance uncertainty = 0.001 g or 0.0005 g Correct expression for % error = (2 × balance uncertainty / mass of residue) × 100