AQA A-Level Chemistry: Atomic Structure & Amount of Substance

The first ionisation energies of the Period 3 elements do not increase smoothly from sodium to argon. The values for the first five elements are shown below.

Element	Na	Mg	Al	Si	P
First ionisation energy / kJ mol⁻¹	496	738	577	786	1012

Describe and explain the trend in first ionisation energy across Period 3, including a clear explanation of why the value falls from magnesium to aluminium and why the value falls from phosphorus to sulfur (the value for sulfur is 1000 kJ mol⁻¹). Refer to nuclear charge, shielding and sub-shell energies in your answer.

(6 marks)

A sample of element X was analysed in a time-of-flight (TOF) mass spectrometer. The spectrum showed three peaks. The relative isotopic mass and percentage abundance of each isotope are given below.

Relative isotopic mass	Percentage abundance / %
24	79.0
25	10.0
26	11.0

(a) Use the data to calculate the relative atomic mass of X. Give your answer to two decimal places and hence identify element X. (4 marks)

(b) The isotope of relative isotopic mass 26 was accelerated in the spectrometer. State the number of neutrons in one atom of this isotope, and explain why this isotope and the isotope of mass 24 have identical chemical properties. (2 marks)

An organic compound Z contains only carbon, hydrogen and oxygen. On analysis it was found to have the following composition by mass.

Element	Carbon	Hydrogen	Oxygen
Percentage by mass / %	40.0	6.7	53.3

The relative molecular mass of Z was found by mass spectrometry to be 90.0.

(a) Use the composition data to determine the empirical formula of Z. Show your working. (3 marks)

(b) Use the empirical formula and the relative molecular mass to determine the molecular formula of Z. (2 marks)

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

In a gas-processing plant, an unknown alkane is recovered as a vapour from a separation column. To identify it, an engineer measures a sample of the vapour and records the following:

Quantity	Value
Mass of vapour sample	2.20 g
Pressure	101 kPa
Volume	1.00 dm³
Temperature	320 K

(a) Use the ideal gas equation $pV = nRT$pV=nRT to calculate the amount, in moles, of gas in the sample. Take $R = 8.314 \ \text{J K}^{-1}\,\text{mol}^{-1}$R=8.314 J K−1mol−1 and convert all quantities to SI units. (3 marks)

(b) Hence calculate the relative molecular mass of the gas and identify the alkane. (2 marks)

Zinc oxide for use in ceramic glazes is manufactured by heating zinc carbonate, which decomposes according to the equation:

$\text{ZnCO}_3(\text{s}) \rightarrow \text{ZnO}(\text{s}) + \text{CO}_2(\text{g})$ZnCO3​(s)→ZnO(s)+CO2​(g)

A batch starts with 12.54 g of zinc carbonate. After heating, 7.32 g of zinc oxide is obtained.

(a) Calculate the percentage yield of zinc oxide. (3 marks)

(b) Calculate the percentage atom economy for the formation of zinc oxide in this reaction. (1 mark)

(Relative atomic masses: Zn = 65.4; C = 12.0; O = 16.0)

In a time-of-flight (TOF) mass spectrometer, a sample is converted into ions which are then separated according to their mass-to-charge ratio.

State the four main stages, in the correct order, by which a time-of-flight mass spectrometer produces a mass spectrum, and briefly state what happens at the acceleration stage. (3 marks)