Waec Chemistry 2002 Past Questions and Answers

Question 56:

• WAEC 2002

(a) Explain in terms of the kinetic theory why a tyre should not be overinflated.

(b)The following results were obtained at room temperature in an experiment to verify one of the gas laws using a glass syringe:

Pressure (P) of air in syringe (atm)	Volume (V) of air in syringe (cm${}^3$	$\frac{I}{V}$
0.100	10.00	0.100
0.125	8.00	0.125
0.150	6.60	0.150
0.175	5.60	0.179
0.200	4.80	0.208
0.225	4.40	0.227

(i) Plot a graph of P against $\frac{1}{v}$, using 1 cm to represent 0.01 atm on the vertical axis and 1cm to represent 0.02 unit on the horizontal axis.
(ii) Which of the gas laws is in agreement with the results?

(c) The flow chart below represents the stages involved in the manufacture of H${}_2$SO${}_4$.
+x +Conc. H${}_2$SO${}_4$ +H${}_2$O
S + O${}_2$ $\to$ SO${}_2$ $\to$ SO${}_3$ $\to$ Y $\to$ Conc H${}_2$SO${}_4$
stage I stage II stage III stage IV

(i) Name the process represented by the chart.
(ii) Identify reactant X and product Y.
(iii) What are the operating temperature and pressure at stage II?
(iv) Mention the stage which requires a catalyst and state the catalyst used.
(v) Give the reason why the SO${}_3$ produced in stage II is not dissolved directly in water to form the acid

(d) When K${}_4$Cr${}_2$C${}_7$ dissolves in water, the following equilibrium is established:
Cr${}_2$O${}_{7(aq)}^{2-}$ + H${}_2$O${}_{(l)}$ $\to$ 2CrO${}_{4(aq)}^{2-}$ + 2H${}_{aq}$
(i) State the colour observed on adding a few drops of dilute H${}_2$SO${}_4$ to the system.
(ii) Explain your answer in (d)(1).

(iii) What principle is applicable to this explanation?

Question 57:

• WAEC 2002

(a)(i) State three methods of preparing salts, giving one example in each case of a salt so prepared.
(ii) What type of salt is each of the following? NaH${}_2$PO${}_4$; (CH${}_3$COO)${}_2$Pb; KAI(SO${}_4$)${}_2$. 12H${}_2$O.

(b)(i) Write an equation for the reaction between dilute HCI and a solution of AgNO${}_3$.
(ii) Explain why NaNO${}_3$ is preferred to AgNO${}_3$ in the preparation of oxygen by thermal decomposition of trioxonitrate (V) salts.
(iii) When silver wire was dipped into an aqueous solution of CuSO${}_4$, the wire remained intact but when the wire was replaced with zinc rod, the rod decreased in size. Give an explanation for this observation.

(c) When a sample of a crystalline salt X was exposed to air, there was a loss in mass.
(i) What phenomenon was exhibited by X?
(ii) Suggest two substances which X could be.
(iii) On heating 5.00 g of a fresh sample of X to constant mass, 1.80g was lost in the form of water vapour. Calculate the number of molecules of water of crystallization in one molecule of X. [H = 1.00; O = 16.00; Anhydrous form of X = 160 g mol${}^{-1}$