Q1 (a) Addition of aqueous copper(II) ions to aqueous iodide ions gives a precipitate of copper(I) iodide and liberates iodine; the iodine can be titrated with aqueous sodium thiosulphate, so these reactions form the basis for a volumetric analysis of copper, for example in metal alloys.
(i) Write an ionic equation for the reaction of copper(II)
ions with iodide ions.
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(ii) Given that the reaction of iodine with sodium
thiosulphate is
I2(aq) + 2S2O32-(aq)
-----> 2I-(aq) + S4O62-(aq)
find the volume of sodium thiosulphate solution of concentration
1.00 mol dm-3 needed to react with the iodine liberated by copper ions from a
brass screw of mass 2.00g, containing 60% of copper by mass.
(4)
(b) Silicon tetrachloride, SiCl4, reacts with
water to give silicon dioxide, SiO2, and HCl; it will react with
atmospheric moisture unless kept in a sealed tube. 1.00g of a poorly-kept
sample of SiCl4 was added cautiously to water. When reaction
was complete the solution was made up to 250cm3 with pure water.
25.0cm3 portions of this required on average 21.6cm3 of
0.100 mol dm-3 sodium hydroxide solution for neutralisation.
What percentage of the sample analysed was SiCl4?
(4)
Q2. The decomposition of sodium azide, NaN3, takes place when
an airbag, which is used as a safety feature in cars, inflates. An airbag
requires a large volume of cool gas to be produced in a few milliseconds.
The gas is produced by the rapid decomposition of the sodium azide.
2NaN3(s) --> 2Na(l) +3N2(g)
When the airbag is fully inflated 54dm3 of nitrogen gas are produced.
(i) Calculate the amount (in moles) of nitrogen gas, N2, produced.
[The molar volume of nitrogen gas under the conditions in the airbag is 20
dm3mol-1]
(1)
(ii) Calculate the amount (in moles) of sodium azide, NaN3, that would be produce 54dm3 of nitrogen gas.
(1)
(iii) Calculate the mass of sodium azide that would produce 54dm3 of nitrogen gas.
(1)