May/June 2026 Paper 51

2026 · 2 questions · 25 parts · 30 marks

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Context for question 1

Solid mixture X contains only anhydrous calcium chloride, CaCl

_{2}

, and anhydrous calcium nitrate, Ca(NO

_{3}

)

_{2}

. An experiment is done to find the percentage by mass of Ca(NO

_{3}

)

_{2}

in the mixture. step 1 Weigh an empty crucible. Add approximately 5 g of mixture X to the crucible. step 2 Reweigh the crucible with mixture X. step 3 Heat the crucible gently for one minute and then strongly for four minutes. The calcium nitrate decomposes.

2 C a (N O_{3})_{2} (s) \to 2 C a O (s) + 4 N O_{2} (g) + O_{2} (g)

step 4 Allow the crucible to cool. Reweigh the crucible and contents.

1(a)Practical Techniques SafetyEasy1 mark

Identify the piece of apparatus that should be used to support the crucible during heating in step 3.

1(b)Practical Techniques SafetyEasy1 mark

Suggest why heating is gentle at first in step 3.

Answer

0 words

1(c)Practical Techniques SafetyEasy1 mark

Give one reason why the heating should be done in a fume hood.

Context for 1(d)

The student carries out the experiment and obtains the results shown in Table 1.1.

Table 1.1

	mass / g
mass of crucible	12.74
mass of crucible + contents before heating	17.81
mass of crucible + contents after heating	16.53

1(d)(i)Mass CalculationsMedium2 marks

Use the data from Table 1.1 to calculate the amount, in mol, of NO

_{2}

gas lost during heating.

amount of NO

_{2}

lost =

0 words

1(d)(ii)Uncertainty Errors EvaluationEasy1 mark

Calculate the percentage error in the measurement of the mass of the crucible + contents before heating. Show your working.

percentage error =

0 words

1(d)(iii)Uncertainty Errors EvaluationMedium1 mark

Other than changing the apparatus, suggest how the percentage error in 1(d)(ii) can be reduced.

1(e)Mass CalculationsMedium2 marks

The experiment is repeated using 4.98 g of mixture X. In this experiment, 0.0223 mol of NO

_{2}

gas is lost during heating. Calculate the percentage by mass of Ca(NO

_{3}

)

_{2}

in this sample of mixture X.

M_{r}

: Ca(NO

_{3}

)

_{2}

, 164.1

percentage by mass of Ca(NO

_{3}

)

_{2}

1(f)(i)Practical Techniques SafetyEasy1 mark

Suggest one additional step that should be done to ensure that the Ca(NO

_{3}

)

_{2}

has completely thermally decomposed.

1(f)(ii)Uncertainty Errors EvaluationMedium1 mark

State the effect on the percentage by mass of Ca(NO

_{3}

)

_{2}

calculated in 1(e) if the Ca(NO

_{3}

)

_{2}

is not completely thermally decomposed. Explain your answer.

1(g)Separation Purification Organic PreparationMedium2 marks

A student wants to collect the NO

_{2}

gas produced by the thermal decomposition of a sample of mixture X by cooling the gas mixture produced until the NO

_{2}

condenses and the oxygen does not.

Table 1.2

	melting point / $^{\circ}$ C	boiling point / $^{\circ}$ C
nitrogen dioxide	$- 11$	21
oxygen	$- 219$	$- 183$

Complete Figure 1.1 to show a labelled diagram of the laboratory apparatus that could be used to prepare liquid NO

_{2}

from mixture X.

Draw with the mouse or a finger.