Past Mock Questions

Question 1

State the colour of phenolphthalein in hydrochloric acid

Answer 1

Colourless

Question 2

State the colour of phenolphthalein in sodium hydroxide

Answer 2

Pink

Question 3

Define a conjugate acid-base pair

Answer 3

A conjugate acid-base pair is any pair consisting of an acid and a base that differ by one proton

Question 4

Define a weak acid in terms of the Bronsted-Lowry theory of acids and bases

Answer 4

A week acid is a poor proton donor

Question 5

Define pH

Answer 5

The pH of a solution is the negative logarithm to the base 10 of a hydrogen ion concentration measured in moles per litre

pH = -log to the base 10 [H+}

Question 6

What are the limitations of the pH scale?
(3)

Answer 6

The pH scale is limited to the 0-14 range even though pH values outside this range are possible - in theory

The pH scale does not work with very concentrated solutions - if the concentration goes above 1M complete dissociation does not always occur - therefore these calculations are not accurate

The pH scale is limited to aqueous solutions and chemical reactions can occur in other types of solution

Question 7

What is an acid according to the Bronsted-Lowry theory of acids and bases

Answer 7

An acid is a proton donor

Question 8

What is a conjugate acid according to the Bronsted-Lowry theory of acids and bases

Answer 8

A base changes into its conjugate acid when it accepts a proton

Question 9

Define Kw the ionic product of water

Answer 9

Kw = Kc[H2O] = [H+][OH-]

Question 10

Write the conjugate acid of H2PO4-

Answer 10

H3PO4

Question 11

Write the conjugate base of H2PO4-

Answer 11

HPO4^2-

Question 12

Define a base according to the Bronsted-Lowry theory

Answer 12

A base is a proton acceptor

Question 13

Write the conjugate acid of HSO4-

Answer 13

H2SO4

Question 14

Write the conjugate base of HSO4-

Answer 14

SO4^2-

Question 15

An acid-base indicator which is itself a weak base (XOH) dissociates according to the following equilibrium expression.

XOH (colourless) —> X+ (pink) + OH-

State and explain the colour change that occurs if an acid is added

Answer 15

If an acid is added then the solution will become pink

This is because the addition of a H+ causes the equilibrium to shift to the right and therefore turn pink

Question 16

An acid-base indicator which is itself a weak base (XOH) dissociates according to the following equilibrium expression.

XOH (colourless) —> X+ (pink) + OH-

State and explain the colour change that occurs if a base is added

Answer 16

If a base is added then the solution will become colourless

This is because the increase in OH- concentration causes the equilibrium to shift to the left and turn colourless

Question 17

Identify one species acting as an base also identify its conjugate acid in the following system

HSO3- + H3O+ –> H2SO3 + H2O

Answer 17

Base = HSO3-
Conjugate acid = H2SO3

Question 18

Explain how an acid-base indicator, which is itself a weak acid and is represented by HX, functions, given it dissociates in water as follows

HX (blue) –> H+ + X- (red)

Answer 18

In an acid the equilibrium lies on the left side which gives the colour of the molecule HX (blue)

In base equilibrium lies on the right side which gives the colour of the ion X- (red)

Question 19

Identify one species acting as a base and also identify its conjugate acid in the following system

H2SO4 + NH3 –> HSO4- + NH4+

Answer 19

Base = NH3
Conjugate acid = NH4+

Question 20

Identify a conjugate pair in the following system

SO3^2- + HCN –> HSO3- + CN-

Answer 20

Acid = HCN
Conjugate base = CN-

Question 21

Identify the species acting as acids in the following system

SO3^2- + HCN –> HSO3- + CN-

Answer 21

HCN
HSO3-

Question 22

Identify one species acting as a base, and also identify its conjugate acid in the following system

H2S + O2- –> OH- +SH-

Answer 22

Base = O2-
Conjugate acid = OH-

Question 23

What indicator would you use for the titration between hydrochloric acid and a sodium carbonate solution

Explain

Answer 23

Methyl orange

Question 24

CH3COOH + NAOH —> CH3COONa +H2O

Name an indicator suitable for this titration

Justify your choice

State the colour change at the end

Answer 24

phenophthalein

Approximate pH range of 8-10 which is suitable for the titration of weak acid against a strong base

Pink to colourless