OCR A-Level Chemistry: Atoms, Moles & Equations

A sample of the element zirconium is analysed in a time-of-flight (TOF) mass spectrometer. The instrument produces a spectrum showing a series of peaks, and from these peaks a value for the relative atomic mass of zirconium is obtained.

Describe and explain how a mass spectrometer is able to produce the data needed to calculate the relative atomic mass of an element such as zirconium. Your answer should refer to what each peak represents, what the height (abundance) of each peak tells you, and how the relative atomic mass is then calculated from the spectrum.

(6 marks)

An organic liquid X, used as a solvent, contains only carbon, hydrogen and oxygen. Combustion analysis gives the percentage composition by mass shown below.

Element	Carbon	Hydrogen	Oxygen
Composition by mass / %	54.55	9.09	36.36

A separate measurement shows that the relative molecular mass of X is 88.0.

Use the data to determine the empirical formula and the molecular formula of X. Show all your working.

(Relative atomic masses: H = 1.0, C = 12.0, O = 16.0.)

(6 marks)

A pure sample of a metalloid element Z was vaporised and analysed in a mass spectrometer. The spectrum contained three peaks. The mass/charge ratio of each peak (for singly charged ions) and its relative abundance are shown below.

m/z	Relative abundance / %
28	90.0
29	6.00
30	4.00

(a) Calculate the relative atomic mass of Z to three significant figures, and hence identify element Z. (3 marks)

(b) Each ion that reaches the detector carries a single positive charge. State the number of protons and the number of neutrons in the ion responsible for the peak at $m/z = 30$m/z=30. (2 marks)

A quality-control technician at a fertiliser plant needs to confirm the formula of a batch of hydrated magnesium sulfate, $\text{MgSO}_4 \cdot x\text{H}_2\text{O}$MgSO4​⋅xH2​O. A sample of the hydrated salt is weighed, then heated strongly until all the water of crystallisation has been driven off, and the remaining anhydrous solid is reweighed.

Measurement	Mass / g
Hydrated salt before heating	4.93
Anhydrous solid after heating	2.41

Use the results to determine the value of x in $\text{MgSO}_4 \cdot x\text{H}_2\text{O}$MgSO4​⋅xH2​O.

(Relative atomic masses: H = 1.0, O = 16.0, Mg = 24.3, S = 32.1.)

(5 marks)

A car airbag is inflated by the rapid decomposition of sodium azide, $\text{NaN}_3$NaN3​, which produces nitrogen gas according to the equation:

$2\text{NaN}_3 \rightarrow 2\text{Na} + 3\text{N}_2$2NaN3​→2Na+3N2​

An airbag module contains 65.0 g of sodium azide. Assume all of it decomposes and that the nitrogen behaves as an ideal gas.

Calculate the volume of nitrogen gas, in dm³, produced at a temperature of 25 °C and a pressure of 100 kPa.

(Relative atomic masses: N = 14.0, Na = 23.0. Gas constant $R = 8.314$R=8.314 J mol⁻¹ K⁻¹. $1 \ \text{m}^3 = 1000 \ \text{dm}^3$1 m3=1000 dm3.)

(4 marks)

This question is about some of the key definitions and equations used when working with amounts of substance.

(a) State what is meant by the relative isotopic mass of an isotope. (1 mark)

(b) When aqueous silver nitrate is added to aqueous sodium chloride, a white precipitate of silver chloride forms. Write the ionic equation, including state symbols, for this reaction. (1 mark)

(c) State the value of the Avogadro constant and what it represents. (1 mark)