1. A is a solution of hydrochloric acid.
B is a solution containing 2.45 g of anhydrous sodium trioxocarbonate (IV) in
250 cm3 of solution.
(a) Put A into the burette and titrate it against 20.0 cm3 or 25.0cm3 portion
of B using methyl orange as indicator. Repeat the exercise to obtain consistent titres. Tabulate your burette readings and calculate the
average volume of A used.
The equation for the reaction involved in the titration is:
Na2CO3(aq) + 2HC1(aq) 2NaC1(aq) + H2O(1) + CO2(g)
[10 marks]
(b) From your results and the information provided, calculate the:
(i) concentration of B in moldm-3;
(ii) concentration of A in moldm-3;
(iii) concentration of A in moldm-3;
(iv) volume of the gas evolved in the reaction at s.t.p.
[H = 1.00; C = 12.0; O = 16.0; Na = 23.0; C1= 35.6;
Molar Volume = 22.4 dm3 mol-1]
[11 marks]
_______________________________________
OBSERVATION
This question was based on acid-base titration. It was attempted by majority of the candidates and their performance was good.
In part (a), candidates correctly tabulated their burette readings and calculated the average volume of acid used using concordant tire values. However, some candidates
lost marks due to alteration of their titre values and wrong averaging of titres.
In the part (b), majority of the candidates correctly calculated the concentration of
A and B in moldm-3. However, some candidates lost marks due to wrong substitution and units. In part (b) (iv) only a few candidates correctly calculated the volume of CO2
evolved in the reaction at s.t.p.
The expected answers include:
Question 1
(a) (i) Titrations: 4 x 2 sets
Averaging(Vcm3)
Na2CO3(aq) + 2HC1(aq) 2NaC1(aq) + H2O + CO2(g)
(b) (i) Conc. of B in moldm-3
Molar mass (Na2CO3) = (2 x 23) + 12 + (3 x 16) + 106 gmol-1
Conc. of B (Na2CO3) = 2.45 = 0.0925 mol dm-3
106 x 0.25
Alt. Method
250 cm 3 of solution contained 2.45g of Na2CO3
1000 cm 3 of solution contained 2.45 x 1000
250
= 2.45×4
= 9.80g
M(Na2CO3) = 106gmol-1
Conc. B(Na2 CO3) = 9.80 =0.0925 moldm-3
106
Conc. of A in moldm-3
From reaction equation
CA VA = 2 CA = 2 x CB x VB
CBVB 1 VB
CA = 2 x 0.0925 x 20/25 = a moldm-3 (correct evaluation to 3 sig. figs)
VA
Conc. of A in gdm-3
M(HC) = 1 + 35.5 = 36.5gmol-1
Conc. of A in (gdm-3) = a x molar mass (HC1)
= a x 36.5
= b gdm-3
(iv) Volume of the gas evolved
From reaction equation
1 mole of Na2CO3 = 1 mole of CO2 at stp
n(CO2) = n (Na2CO3) = 0.0925 mole
… Vco2 = n x Vm
= 0.0925 x 22.4dm3
= 2.07dm3 at stp
… Vco2 liberated in 25 cm3 of Na2CO3 will be 2,07 x 25 x 25
1000
= 0.0518 dm3/51.80 cm3 at stp
========
Question 2
C contains two cations and two anions. Perform the following exercises
on C. Record your observations and identify any gas(es) evolved. State the
conclusion you draw from the result of each test.
(a) Dissolve all of C in about 10 cm3 of distilled water:
Stir the resulting solution thoroughly.
(i) To about 2cm3 of the solution, add few drops of
AgNO3 solution, followed by HNO3(aq).
(ii) To another 2cm3 portion of the solution, add dil. HC1
followed by BaC12 solution.
(iii) To another 2 cm3 portion of the solution, add NaOH(aq)
dropwise and then in excess. Warm the mixture.
(iv) To another 2cm3 portion of the solution, add NH3(aq)
dropwise and then in excess.
[20 marks]
_______________________________________
OBSERVATION
This question is based on qualitative analysis. It involved the identification of cations and onions.
The question was attempted by majority of the candidates and their performance was fair.
Majority of the candidates carried out the required text, made their observation and inferences logically.
Some candidates lost marks due to wrong test, observation, inference or the used of wrong charges on ions.
The expected answers include:
==========
Question 3
Consider the following experimental set-up.
(i) Identify by name: P; Q; R; S and T.
(ii) State the method of collection of gas, S.
(iii) What is the function of R in the experimental set-up?
(iv) Write the balanced equation of the reaction for the
preparation of gas, S.
[9 marks]
_______________________________________
OBSERVATION
This question is on theory of practical work. Majority of the candidates
that attempted this question did poorly.
Some of the candidates lost marks because they used formulae in identifying
the components instead of names.
Also, some candidates used wrong lettering of symbols of elements in writing out the equation. e.g. instead of ‘Cl’ some candidates wrote ‘CL’ and ‘NACL’ instead of
‘NaCl’ etc.
The expected answers include:
(i) P – Delivery tube
Q – Drying tower
R – Calcium oxide (accept quicklime)
S – Ammonia gas
T – Retort stand with clamp
(ii) Upward delivery (of gas)/ downward displacement of air
(iii) To dry the gas
(iv) Ca(OH)2(aq) + 2NH4Cl(aq) → CaCl2 + 2H2O + 2NH3(g).
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