WAEC/NECO CHEMISTRY PAPER 3 GCE PAST QUESTIONS AND ANSWERS AS REQUESTED

WAEC/NECO CHEMISTRY PAPER 3 GCE PAST QUESTIONS AND ANSWERS AS REQUESTED

QUESTION 1

In a titration experiment, 22.50 cm3 of an acid solution A containing 10.6 g of NaHSO4 per dm3 reacted with 25.0 cm3 of solution B containing X g of NaOH per dm3. The equation for the reaction is:

(a) From the information given above, calculate the:
(i) concentrate of A in moldm– 3;
(ii) concentrate of B in moldm– 3;
(iii) value of X;
(iv) mass of Na2SO4 formed during the reaction. [ H = 1.00; 0 = 16.0; Na = 23.0; S = 32.0] (16 marks)

(b) (i) Name a suitable indicator for the titration experiment.
(ii) State the apparatus used to measure the volume of solution: I. A; II B; (3 marks)

In a titration experiment, 22.50 cm3 of an acid solution A containing 10.6 g of NaHSO4 per dm3 reacted with 25.0 cm3 of solution B containing X g of NaOH per dm3. The equation for the reaction is: NaHSO4(aq) + NaOH(aq) Na2SO4(aq) + H2(l)

(a) From the information given above, calculate the:

(i) concentrate of A in moldm– 3;
(ii) concentrate of B in moldm– 3;
(iii) value of X;
(iv) mass of Na2SO4 formed during the reaction. [ H = 1.00; 0 = 16.0; Na = 23.0; S = 32.0] (16 marks)

(b)

(i) Name a suitable indicator for the titration experiment.
(ii) State the apparatus used to measure the volume of solution: I. A; II B; (3 marks)

ANSWER 1

This question was attempted by majority of the candidates and their performance was fair.

In (a), majority of the candidates lost mark for writing the unit of molar mass in “g” instead of in “g mol”. Although majority of the candidates were able to calculate the concentration of both the acid and base in moldm– 3, a lot of candidate lost marks for not expressing their answers to 3 significant figures.

In (b), candidates were able to name suitable indicators for the titration and they were also able to name the apparatus used to measure the volume of the solution of the acid and base.

The required answers include: (a) (i) Molar mass NaHSO4 = [23.0 + 1.00 + 32.0 + 64.0] = 120gmol-1 (No score for wrong Unit)
Conc. of A = 10.6/120
= 0.0883mol dm-3 (correct evaluation to 3sig.figure) (No score for wrong Unit).

(ii) Reaction: NaHSO4(aq) + NaOH(aq) ? Na2SO4(aq) + H2O(l)
From the reaction equation CAVA / CBVB = 1 /1
CB = CAVA / VB
CB = (0.0883 X 22.5) / 25.0= 0.0795mol dm-3(No score for wrong Unit)

iii) Molar mass of NaOH = (23.0 + 16.0 + 1.00) = 40gmol-1 (No score for wrong Unit)
Mass of NaOH gdm-3 = 40 x 0.0795
= 3.18gdm-3
Value of X = 3.18

(iv) Amount of NaOH in 25.0cm3 of B
= (25.0 x 0.0795)/1000
= 0.00199 mol
From the reaction equation
1 mol NaOH gives 1 mol Na2SO4
0.00199 mol NaOH = 0.00199 mol Na2SO4

Molar mass of Na2SO4 = (2 x 23.0) + 32.0 + (4 x 16.0)
= 142gmol-1
Mass of Na2SO4 = 142 x 0.00199
= 0.283g

Alternative Method 2 to (iv) Mass of NaOH in 25.0cm3 of 0.0795moldm-3
m = MCV
= 40 x 0.0795 x 25 / 1000
= 0.0795g
Molar Mass of Na2SO4 = 142gmol-1
From the balanced equation:
40g NaOH = 142g of Na2SO4
0.0795g NaOH = Xg Na2SO4
mass of Na2SO4 = 0.0795 x 142 g / 40
= 0.282g

(b) (i) Phenolphtalein (Accept methyl orange)
(ii) I – Burette
II – Pipette

QUESTION 2

Question 2 
D is an inorganic salt. E is a solution of an inorganic salt. The tests recorded in the table below were performed on D and E.Copy and complete the table.

Test Observation Inference
(a) D + distilled water Did not dissolve ……………………..
(b) D + dil. HCI + heat Dissolved to form blue or greenish-blue solution …………………….
(c) Solution of D
+ NaOH(aq) in drops then in excess
Blue gelatinous precipitate
…………………………
……………………..
(d) Solution of D
+ NH3(aq) in drops then in excess
…………………………..
……………………………
Cu2+ present
Cu2+ confirmed
(e) (i)  E(aq) + NaOH(aq) in    drops and in excess No precipitate ………………….
(ii)Mixture in 2(e)(i) warmed ……………………………
…………………………..
…………………………..
NH3 gas evolved, therefore NH4+ present
(f) E(aq)  +BaCl2(aq) White chalky precipitate.  No gas evolved, precipitate did not dissolve …………………….
…………………….
……………………

In a titration experiment, 22.50 cm3 of an acid solution A containing 10.6 g of NaHSO4 per dm3 reacted with 25.0 cm3 of solution B containing X g of NaOH per dm3. The equation for the reaction is: NaHSO4(aq) + NaOH(aq) Na2SO4(aq) + H2(l)

(a) From the information given above, calculate the:

(i) concentrate of A in moldm– 3;
(ii) concentrate of B in moldm– 3;
(iii) value of X;
(iv) mass of Na2SO4 formed during the reaction. [ H = 1.00; 0 = 16.0; Na = 23.0; S = 32.0]    (16 marks)

(b)

(i) Name a suitable indicator for the titration experiment.
(ii) State the apparatus used to measure the volume of solution: I. A; II B;   (3 marks)

Answers to a and b to be provided later

_____________________________________________________________________________________________________
ANSWER 2
The candidate made a fair attempt on this question. Majority of the candidates obeyed the instruction “copy and complete”, thus was commendable. Some candidates still cannot differentiate between gelatinous and chalky precipitate. Results of test in drops and excess still seem confusing to some candidates as some of them gave the result together (i.e. only in excess).The expected answers include:

   Test  Observation  Inference
(a) D + distilled water Did not dissolve D is an insoluble salt
(b) D + dil. HCl + heat Dissolved to form blue or greenish blue solution Cu2+  may be present
(c) Solution of D
+ NaOH(aq) in drops
then in excess
Blue gelatinous precipitate.
Precipitate insoluble
Cu2+ present
(d) Solution of D 
+ NH3(aq) in drops
then in excess
 Blue gelatinous Precipitate 
Precipitate dissolves forming deep blue solution
Cu2+ present
Cu2+ present confirmed
(e)(i) E(aq) + NaOH(aq) in drops and in excess No precipitate NH4+may be present
  (ii) Mixture in 2(e)(i) warmed Colourless gas evolved 
Pungent/choking/irritating smell turns red litmus to blue/forms dense white fume with conc. HCl
NH3 gas evolved
NH4+  present
(f) E(aq) + BaCl2(aq)
+ excess HCl(aq)
White chalky precipitate.
No gas evolved
Precipitate did not dissolve
CO32-SO32- SO42-
SO42-   Present
(SO42- must have been mentioned above)
QUESTION 3 
(a) (i) Draw a labelled diagram of a simple set-up that can be used to distinguish between an electrolyte and a non-electrolyte.
(ii) What is the name of the set-up drawn in 3(a)(i)?
(iii) Name one: I. electrolyte; II. non-electrolyte. (7 marks)
(b) Name three pieces of apparatus that could be used in determining the solubility of a named salt in the laboratory.
(3 marks)
(c) (i) Name one reagent that could be used to distinguish between the following ions in qualitative analysis: Ca2+; Pb2+; Zn2+ and Al3+ .
(ii) State the observation that could be made in each case with the named reagent in 3(c)(i). (5 marks)
(d) Explain briefly the term end-point. (2 marks)
_____________________________________________________________________________________________________
ANSWER 3
The question was poorly attempted by majority of the candidates. In (a), only a few number of candidates were able to draw a labelled diagram of a simple electrolytic cell. Also, some candidates lost marks in (a)(iii) for giving formulae instead of names. Question (c), was poorly attempted. Very few candidates could distinguish between cation in qualitative analysis. Most candidates could not distinguish the observation in using NaOH(aq) and NH3(aq). In (d), candidates found it difficult to explain the term endpoint.The expected answers include:(a) (i)
(ii) Electrolytic cell
(iii) I- Tetraoxosulphate (VI) acid – sodium chloride solution 
II – Glucose solution
– Sugar solution
– Petrol/Kerosene

(b) – Evaporating dish/Beaker
– Tripod stand and wire gauze
– Bunsen burner
– Burette/Pipette
– Glass rod / stirrer
– Thermometer
– Weighing balance/Scale

(c) (i) Ammonia solution 
(ii) Ca2+ – No precipitate with NH3(aq)
Pb2+ – White chalky precipitate insoluble in excess
Zn2+ – White gelatinous precipitate soluble in excess
 
Al3+ – White gelatinous precipitate insoluble in excess

(d) End point is a stage/point in a titration where a given amount of reagent in a solution (acid oxidizing agent) completely react with another reagent (base reducing agent). It is usually indicated by colour change.

ALL QUESTIONS AND COMMENTS PROVIDED BY WAEC

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