THE ELEMENTS OF GROUP III 159
(a) Magnesium chloride is a high melting-point solid, aluminium
chloride is a solid which sublimes readily at about 480 K, and
silicon tetrachloride is a volatile liquid. Explain the nature of
the chemical bonding in these chlorides and show how this
accounts for the above differences in volatility.
(b) Explain why the freezing point of an aqueous solution of
sodium hydroxide is unchanged when aluminium oxide is
dissolved in the solution.
(C,A)
- Describe the laboratory preparation, from aluminium, of (a)
anhydrous aluminium chloride, (b) potassium aluminium sulphate
dodecahydrate.
Why is potassium aluminium sulphate not soluble in benzene? A
compound M has the composition C = 50.0%; H=12.5%;
Al = 37.5%. 0.360 g of M reacts with an excess of water to evolve
0.3361 of gas N and leave a white gelatinous precipitate R. R
dissolves in aqueous sodium hydroxide and in hydrochloric acid.
20cm^3 of N require 40cm^3 of oxygen for complete combustion,
carbon dioxide and water being the only products. Identify com-
pounds N and R, suggest a structural formula for M, and write an
equation for the reaction of M with water. (All gas volumes were
measured at s.t.p.)
[H = 1.0; C = 12.0; O = 16.0; Al - 27.0; molar volume of a
gas = 22.41 at s.t.p.]
(C,A)
