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2018 Waec Chemistry Practical Expo.


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PLS NOTE THAT YOU ARE REQUIRED TO
USE UR SCHOOL’S TITRE VALUE IN
NUMBER 1.
OUR SCHOOL GOT 15.30CM3.
REPLACE WITH UR SCHOOLS OWN AND
RECALCULATE.

(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+

=========================================

(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

=========================================

(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)


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