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WAEC Chemistry Past Questions & Answers - Page 14

66.

What is the percentage composition of carbon
in Ca(HCO3) $_{2}$ ?
[ Ca = 40.0, 0= 16.0, C = 12.0, H= 1.0]

22.2%

14.8%

7.4%

3.7%

View answer & explanation

Correct answer is B

. % of C in Ca(HCO3)2
Molar mass of Ca(HCO3)2 = 40+(1+12+(16*3))2
=40+(13+48)2
=40+122
=162
C/Ca(HCO3)2 x 100
=12 * 2/162 * 100
=14.8%

67.

Which of the following elements would displace copper from a solution of copper ions?

silver

gold

lead

mercury

View answer & explanation

Correct answer is C

The reactivity series allows us to predict how metals react. A more reactive metal displaces a less reactive metal from a compound. Lead is more reactive metal than copper hence, copper is displaced from copper solution by lead.

68.

If 50 cm $^{3}$ of a saturated solution of KNO $_{3}$ at 40 °C contained 5.05 g of the salt, its solubility at the same temperature would be
[KNO $_{3}$ = 101]

1.0 mol dm $^{-3}$

1.5 mol dm $^{-3}$

2.0 mol dm $^{-3}$

5.0 mol dm $^{-3}$

View answer & explanation

Correct answer is A

V= 50cm $^{3}$
Mass= 5.05g
Relative molecular mass of KNO $_{3}$ = (39+14+(3*16)) = 101
Convert 50cm $^{3}$ to dm $^{3}$ which is
1000cm³ = 1dm $^{3}$
50cm³ = 50*1/1000
= 0.05dm $^{3}$
Moles = mass/ molar mass
= 5.05/101 =0.05mole
Solubility= mole/volume
Solubility=0.05mol/0.05dm $^{3}$
Solubility=1.0mol/dm_ $^{3}$