WAEC 2018 CHEMISTRY PRACTICAL ANSWERS – MAY/JUNE – wakagist.com

(1a)

Volume of pipette used=25.0cm^3

TABULATE

Reading: Rough|1st|2nd|3rd

Final reading: 16.00|15.40|17.20|15.30

Initial reading: 0.00|0.00 |2.00 |0.00

Vol of acid used: 16.00|15.40|15.20|15.30

Average volume of A used = (VA1 + VA2 + VA3) cm³

VA = (15.40 + 15.20 + 15.30)/3cm³

VA = 45.90/3 = 15.30cm³.

B contains 4.80g/250cm³

Hence x8 = 1000cm³

X = 4.80 × 1000/250 = 4.80 × 4 = 19.2gdm³

(1bi)

Hence molar concentration of B in moldm3

Mass Conc(8dm³) = molar Conc × molar mass

Molar Conc of B = mass conc/molar mass = 19.2gdm³/127gmol

Cb = 0.151moldm3

(1bii)

Concentration of A in moldm-³, CA is given by CAVA/CBVB = na/nb

Where CA = ?

VA = 15.30cm³,

na = 1

CB = 1.51moldm-³, VB = 25.0cm³, nb = 5

CA × 15.30/0.151 × 250= 1/5

Therefore CA = 0.151×25×1/15.30×5 = 3.775/76.50 = 0.049moldm-³

Hence Conc. Of A in moldm-³ = 0.049moldm-³

(1biii)

No of moles of Fe²+ in the volume of B pipetted

n= cv/1000 =

0.151×25.0/1000 =

3.775/1000

= 0.003775moles of Fe²+

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(2a)
TEST: C + about 5cm3 of distilled water
OBSERVATION: C dissolves to form a clear solution;
Turns blue litmus paper to pink
INFERENCE: C is a soluble salt;
C is fairly acidic

(2ai)
TEST: 1st portion of C + NaOH(aq) in drops;
then excess
OBSERVATION: pale-blue precipitate forms in drops of NaOH(aq) which persists in excess of NaoH(aq)
INFERENCE: cu²+ present.

(2aii)
TEST: 2nd portion of C + NH3(aq) in drops and then in excess
OBSERVATION: pale blue precipitate forms in drops of NH3(aq).
Ppt dissolves to form a deep blue coloration.
INFERENCE: Cu²+ present.
Cu²+ confirmed

(2aiii)
TEST: 3rd portion of c + AgNO3(aq) + HCL(aq)
OBSERVATION: a creamy white ppt forms.
INFERENCE: Halogen/Halides present.

(2bi)
TEST: D + about 5cm³ of distilled H2O
OBSERVATION: D dissolves to form a clear solution.
The test tube feels warmth
INFERENCE: D is a soluble salt.

(2bii)
TEST: 2cm³ of D + HCL(aq)
OBSERVATION: effervescence occurs, an odourless and colourless gas evolved. Precipitate dissolves in dil HCL(aq)
INFERENCE: CO32-, so32-present
CO32- present

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(3a)
A white precipitate of barium sulphate will be formed by the instant reaction between sulphuric acid and barium chloride.
H2SO4(aq) + BaCl2(aq) -> BASO4 + 2HCl

(3b)
Iron II sulphide reacts with hydrochloric acid, releasing the malodorous and very toxic gas, hydrogen sulphide
FeS(aq) + 2HCl -> Fecl2(aq) + H2S(g)

(3c)
When solid iron fillings are added to dilute aqueous hydrochloric acid, Iron(II)Chloride OR ferrous chloride is formed, with the liberation of hydrogen gas.
Fe(s) + 2HCL(aq) -> Fecl2 + H2(g)

COMPLETED

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