1. (a) 0.100 g of magnesium ribbon is added to 50.0 cm3 of 1.00 mol dm–3 sulfuric acid to produce hydrogen gas and magnesium sulfate.

Mg (s) + H2SO4 (aq)→H2(g) + MgSO4(aq)

(i) The graph shows the volume of hydrogen produced against time under these experimental conditions.

Sketch two curves, labelled I and II, to show how the volume of hydrogen produced (under the same temperature and pressure) changes with time when:

  • I. using the same mass of magnesium powder instead of a piece of magnesium ribbon;
  • II. 0.100 g of magnesium ribbon is added to 50 cm3 of 0.500 mol dm–3 sulfuric acid.

(ii) Outline why it is better to measure the volume of hydrogen produced against time rather than the loss of mass of reactants against time.

(b) Magnesium sulfate can exist in either the hydrated form or in the anhydrous form. Two students wished to determine the enthalpy of hydration of anhydrous magnesium sulfate. They measured the initial and the highest temperature reached when anhydrous magnesium sulfate, MgSO4(s), was dissolved in water. They presented their results in the following table.

(i) Calculate the amount, in mol, of anhydrous magnesium sulfate.

(ii) Calculate the enthalpy change, ΔH1, for anhydrous magnesium sulfate dissolving in water, in kJ mol–1. State your answer to the correct number of significant figures.

(c) The students repeated the experiment using 6.16 g of solid hydrated magnesium sulfate,MgSO4•7H2O (s), and 50.0 cm3 of water. They found the enthalpy change, ΔH2 , to be +18 kJ mol−1.

The enthalpy of hydration of solid anhydrous magnesium sulfate is difficult to determine experimentally, but can be determined using the diagram below.

(i) Determine the enthalpy change, ΔH, in kJ mol–1, for the hydration of solid anhydrous magnesium sulfate, MgSO4.

(ii) The literature value for the enthalpy of hydration of anhydrous magnesium sulfate is –103 kJ mol–1. Calculate the percentage difference between the literature value and the value determined from experimental results, giving your answer to one decimal place. (If you did not obtain an answer for the experimental value in (c)(i) then use the value of –100 kJ mol–1, but this is not the correct value.)

(d) Another group of students experimentally determined an enthalpy of hydration of –95 kJ mol–1. Outline two reasons which may explain the variation between the experimental and literature values.

(e) Magnesium sulfate is one of the products formed when acid rain reacts with dolomitic limestone. This limestone is a mixture of magnesium carbonate and calcium carbonate.

(i) State the equation for the reaction of sulfuric acid with magnesium carbonate.

(ii) Deduce the Lewis (electron dot) structure of the carbonate ion, giving the shape and the oxygen-carbon-oxygen bond angle.

Lewis (electron dot) structure:


Bond angle:


2. Magnesium has three stable isotopes, 24Mg, 25Mg and 26Mg. The relative abundance of each isotope is 78.99 %, 10.00 % and 11.01 % respectively, and can be determined using a mass spectrometer.

(a) Describe the processes occurring at stage Q and stage S.

(b) (i) Define the term relative atomic mass.

(ii) Calculate, showing your working, the relative atomic mass, Ar , of magnesium, giving your answer to two decimal places.




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