WAEC Chemistry Past Questions & Answers - Page 16

76.

Consider the following reaction equation
N\(_{2}\)(g) + 3H\(_{2(g)}\)  ⇌ 2NH3(g) ∆H = -92kJ

Increasing the temperature of the reaction would

A.

shift the equilibrium to the right

B.

increase the yield of ammonia

C.

decrease the amount of hydrogen

D.

decrease the yield of ammonia

Correct answer is D

In this case, an increase in in temperature will cause the equilibrium position to shift to the left thereby decreasing the yield of ammonia.

77.

Which of the following statements about an equilibrium system is correct?

A.

Forward and backward reactions occur at the same rate

B.

The concentrations of reactants must equal that of the products

C.

The concentrations of, reactants and products can be changed by adding a catalyst,

D.

The concentrations of reactants and products are not affected by a change in temperature

Correct answer is A

Equilibrium is a state of a system where there is no observable change in the properties of the system with respect to time

78.

Which of the following gases is alkaline?

A.

NO\(_{2}\)

B.

CO\(_{2}\)

C.

NH\(_{3}\)

D.

N\(_{2}\)O

Correct answer is C

Alkaline gases are any gaseous compounds that form an alkali (or basic) solution with a pH greater than 7 when dissolved in water. Ammonia is the most common alkaline gas

79.

Graphite is used as a dry lubricant due to the presence of

A.

mobile electrons

B.

free electrons

C.

octahedral structures

D.

layered structures

Correct answer is D

Graphite is an excellent dry lubricant. This is because its layered structure allows one layer to slide over another easily. It is non-volatile and not sticky.

80.

What is the partial pressure of oxygen at s.t.p. in a gaseous mixture containing 100 cm3 of oxygen and 900 cm3 of nitrogen gas?

A.

1.0 atm

B.

0.1 atm

C.

9.0 atm

D.

0.9 atm

Correct answer is B

Partial pressure is found by Dalton's Law:
Px = PTotal ( nx / nTotal )
where
Px = partial pressure of gas x
PTotal = total pressure of all gases
nx = number of moles of gas x
nTotal = number of moles of all gases


Step 1
Find PTotal
Standard pressure is 1 atm.


Step 2
Add up the number of moles of the component gases to find nTotal
nTotal = noxygen + nnitrogen
nTotal = 100 + 900
nTotal =1000cm3


Step 3
Now you have all the information needed to plug the values into the equation and solve for Poxygen
P-oxygen = PTotal ( n-oxygen / nTotal )
oxygen = 1 atm ( 100 / 1000 )
oxygen= 0.1atm