WAEC Chemistry Past Questions & Answers - Page 15

71.

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]

A.

1.0 mol dm\(^{-3}\)

B.

1.5 mol dm\(^{-3}\)

C.

2.0 mol dm\(^{-3}\)

D.

5.0 mol dm\(^{-3}\)

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_\(^{3}\)

72.

What number of moles of oxygen would exert a pressure of 10 atm at 320 K in an 8.2 dm\(^{3}\) cylinder?
[R=0.082 atm dm\(^{-3}\) mol\(^{-1}\) K\(^{-1}\)]

A.

0.32

B.

1.52

C.

3.13

D.

31.25

Correct answer is C

P = 10atm, T= 320K, V= 8.2dm\(^{3}\)
PV =nRT
n = P * V/R * T
10 * 8.2/0.082 * 320
= 82/26.24
n = 3.13

73.

A hydrogen chloride gas reacted with oxygen gas to yield water and chlorine gas. The mole ratio of the hydrogen chloride gas to water is

A.

1:3

B.

2:1

C.

3:1

D.

4:1

Correct answer is B

4HCl + O\(_{2}\) -- 2Cl\(_{2}\) + 2H\(_{2}\)O
4:2 = 4/2
2:1

74.

Which of the following statements about liquids is/are true?


I. Liquids maintain their volume at constant temperature.
II. Liquids have fixed shape
III. Liquids do not diffuse
IV. Change in pressure affects volume of liquids

A.

I only

B.

IV only

C.

I and IV only

D.

II and III only

Correct answer is A

A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure

75.

When air in a syringe is compressed such that there is no change in temperature, the

A.

air liquefies

B.

pressure increases

C.

intermolecular space increases

D.

density decreases

Correct answer is B

Since the temperature of air remain constant.So, it is a isothermal process.
PV=K
This eq. shows that on increasing pressure, the volume gets reduced and vice-versa.
Since the syringe is slowly compressed, volume is decreasing.
Therefore, the pressure inside the syringe will increases because the molecules now hit the syringe walls more frequently.