Question 2
- C is a mixture of two inorganic salts. The following tests were carried out on C.
Copy and complete the table.
|
Test |
Observation |
Inference |
(a) |
C(s) + distilled water. Stirred then filtered |
C partly dissolved to give colourless filtrate and a white residue |
………………. |
(b)i |
(i) Portion of filtrate + NaOH(aq) |
……………………… |
NH3(g) from |
Ii |
Glass rod dipped in conc. HCl in contact with the gas |
……………………… |
NH4+ confirmed |
(c) |
Portion of filtrate +BaCl2(aq) |
White precipitate |
SO42- ............ |
(d) |
Portion of residue + HCl(aq) |
Effervescence |
CO2(g) from |
E(e) |
(i) Portion of solution from 2(d) |
white precipitate |
………………..
……………….. |
|
(ii) Potion of solution from 2 (d) + H2SO4(aq) in excess |
…………………….. |
Ca2+ confirmed |
(f) Name the compound in the:
(i) filtrate;
Observation
In part (a), majority of the candidates could not write correct inference.
In part (b), majority of the candidates did not write no visible reaction when reporting their observation. Consequently, they lost the mark allocated to the question.
In part (c), majority of the candidates did not indicate ‘precipitate insoluble’.
In part (d), majority of the candidates omitted the words “odourless gas”.
In part (e), candidates could not differentiate the white precipitate of Zn2+, Al3+ from that of Pb2+ and Ca2+. They listed the ions interchangeably.
In part (f), candidates could not name the compound in the filtrate and the residue.
The expected answers include:
|
|
TEST |
OBSERVATION |
INFERENCE |
a |
|
C(s) + distilled water |
C partly dissolved to give coluorless filtrate and a white residue |
C contains soluble and insoluble salts |
b |
(i) |
Portion of filtrate |
No Visible Reation |
NH3(g) from NH4+ present |
|
(ii) |
Glass rod dipped |
White dense fume |
NH4+ Confirmed |
c |
|
Portion of filtrate + excess HCl (aq) |
White precipitate Ppt. insoluble |
SO4² ̄ ;SO3² ̄ ;CO3² ̄present SO42 ̄ confirmed |
d |
|
Portion of residue |
Effervescence of Colourlessodourless gas . Turns blue litmus paper red). Turns lime water milky |
CO2(g) from |
e |
(i) |
Portion of solution from |
White precipitate |
++ |
|
(ii) |
Portion of solution from |
White precipitate |
Ca²+ confirmed |
f. (i) Filtrate: Ammonium tetraoxosulphate (VI)
(ii) Residue: Calcium trioxocarbonate (IV)