Chemistry Past Paper -
June 2009

1 The Group 2 element magnesium was first isolated by Sir Humphry Davy in 1808.
(a) Magnesium has three stable isotopes, which are 24Mg, 25Mg and 26Mg.
(i) Complete the table below to show the atomic structures of 24Mg and 25Mg.

24Mg - protons neutrons electrons
25Mg - protons neutrons electrons

(ii) A sample of magnesium contained 24Mg: 78.60%; 25Mg: 10.11%; 26Mg: 11.29%.
Calculate the relative atomic mass of this sample of Mg.
Give your answer to four significant figures.

(iii) Define the term relative atomic mass.

(b) The reaction between magnesium and sulfuric acid is a redox reaction.
Mg(s) + H2SO4(aq) MgSO4(aq) + H2 (g)
(i) Use oxidation numbers to identify which element has been oxidised.
Explain your answer.

(ii) Describe what you would see when magnesium reacts with an excess of sulfuric acid.

(c) Epsom salts can be used as bath salts to help relieve aches and pains.
Epsom salts are crystals of hydrated magnesium sulfate, MgSO4•xH2O.
A sample of Epsom salts was heated to remove the water. 1.57 g of water was removed
leaving behind 1.51 g of anhydrous MgSO4.
(i) Calculate the amount, in mol, of anhydrous MgSO4 formed.

(ii) Calculate the amount, in mol, of H2O removed.

(iii) Calculate the value of x in MgSO4•xH2O.

2 This question compares the bonding, structure and properties of sodium and sodium oxide.
(a) Sodium, Na, is a metallic element.
Explain, with the aid of a labelled diagram, what is meant by the term metallic bonding.

(b) Sodium reacts with oxygen to form sodium oxide, Na2O, which is an ionic compound.
(i) Write the equation for the reaction of sodium with oxygen to form sodium oxide.

(ii) State what is meant by the term ionic bond.

(iii) Draw a ‘dot-and-cross’ diagram to show the bonding in Na2O.

(c) Compare and explain the electrical conductivities of sodium and sodium oxide in the solid and liquid states.