Chemistry Paper 3B, Nov/Dec 2015

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) + heat	……………………… ……………………… ……………………….	NH3(g) from NH4+ present
Ii	Glass rod dipped in conc. HCl in contact with the gas	………………………	NH4+ confirmed
(c)	Portion of filtrate +BaCl2(aq) + excess HCl(aq)	White precipitate ……………………..	SO42- ............ ……… present SO42- confirmed
(d)	Portion of residue + HCl(aq)	Effervescence ……………………. ……………………. …………………….	CO2(g) from CO32-
E(e)	(i) Portion of solution from 2(d) + NaOH (aq) In drops, then in excess	white precipitate insoluble in excess	……………….. ………………..
	(ii) Potion of solution from 2 (d) + H2SO4(aq) in excess	……………………..	Ca2+ confirmed

(f) Name the compound in the:
(i) filtrate;

(ii) residue. [18 marks]

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

C(s) + distilled water
Stirred then filtered.

C partly dissolved to give coluorless filtrate and a white residue

C contains soluble and insoluble salts

(i)

Portion of filtrate
+NaOH(aq)
+ Heat

No Visible Reation
Colourless gas Pungent/ irritating/ Choking gas Turns red litmus paper blue

NH3(g) from

NH4+ present

(ii)

Glass rod dipped
in conc. HCl in contact with the gas

White dense fume

NH4+ Confirmed

Portion of filtrate
+BaCl2

+ excess HCl (aq)

White precipitate

Ppt. insoluble

SO4² ̄ ;SO3² ̄ ;CO3² ̄present

SO42 ̄ confirmed

Portion of residue
+HCl(aq)

Effervescence of Colourlessodourless gas . Turns blue litmus paper red). Turns lime water milky

CO2(g) from
CO3² ̄

(i)

Portion of solution from
(d)+NaOH(aq) in drops
then in excess

White precipitate
Insoluble in excess

++
Ca2+,Pb2+
Ca2+ Present

(ii)

Portion of solution from
(d)+H2SO4 in excess

White precipitate

Ca²+ confirmed

f. (i) Filtrate: Ammonium tetraoxosulphate (VI)
(ii) Residue: Calcium trioxocarbonate (IV)

Chemistry Paper 3B Nov/Dec 2015

Question 2

Observation