A LevelChemistryOCRExam Questions5. Physical Chemistry & Transition ElementsAcids, Bases & Buffers

Acids, Bases & Buffers (OCR A Level Chemistry A): Exam Questions

Exam code: H432

4 hours37 questions

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Acids, Bases & Buffers

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1
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1 mark

The pH of hydrochloric acid, HCl (aq), with a concentration of 0.15 mol dm-3 is

  • 0.15

  • 0.82

  • -0.82

  • 1.00

Choose your answer

2
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1 mark

What is the pH of a 0.008 mol dm-3 solution of sodium hydroxide, NaOH, at 25 °C?

(Kw = 1.00 x 10-14 mol2 dm-6 at 25 °C)

  • 2.10

  • 10.90

  • 11.90

  • 12.90

Choose your answer

3
1 mark

What type of titration is represented by the pH curve below? 

Graph showing pH change from 13 to 3 as acid volume increases from 0 to 50 cm³, with a steep drop around 25 cm³.

  • Strong alkali being added to weak acid

  • Strong acid being added to weak alkali

  • Weak acid being added to strong alkali

  • Weak acid being added to weak alkali

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4
1 mark

Which statement about buffers is correct?

  • A buffer resists small changes in pH

  • A buffer can be formed from sodium hydroxide and an excess of ethanoic acid

  • Buffers are able to resist changes in pH when strong acids and alkalis are added

  • If a strong acid is added to a buffer solution the concentration of the weak acid decreases

Choose your answer

5
1 mark

Which Ka expression represents the dissociation of benzoic acid, C6H5COOH?

  • K subscript a space equals space fraction numerator open square brackets straight C subscript 6 straight H subscript 5 COOH close square brackets over denominator open square brackets straight H subscript blank superscript plus close square brackets open square brackets straight C subscript 6 straight H subscript 5 COO subscript blank superscript minus close square brackets end fraction

  • K subscript a space equals space fraction numerator open square brackets straight H subscript blank superscript plus close square brackets open square brackets straight C subscript 6 straight H subscript 5 COO subscript blank superscript minus close square brackets over denominator open square brackets straight C subscript 6 straight H subscript 5 COOH close square brackets end fraction

  • K subscript a space equals space open square brackets straight H subscript blank superscript plus close square brackets open square brackets straight C subscript 6 straight H subscript 5 COO subscript blank superscript minus close square brackets

  • K subscript a space equals space open square brackets straight C subscript 6 straight H subscript 5 COOH close square brackets

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6
1 mark

The Ka and pKa values for four acids are given below.

Acid

Ka / mol dm⁻³

pKa

Methanoic acid

1.8 × 10-4

3.75

Ethanoic acid

1.7 × 10-5

4.76

Benzoic acid

6.3 × 10-5

4.20

Hydrocyanic acid

4.9 × 10-10

9.31

Which is the weakest acid?

  • Methanoic acid

  • Ethanoic acid

  • Benzoic acid

  • Hydrocyanic acid

Choose your answer

1
1 mark

Which statement is correct? 

  • As temperature increases, the pH value of pure water decreases

  • As temperature decreases, the pH value of pure water decreases

  • The pH of water is unaffected by temperature

  • Pure water is not neutral

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2
1 mark

Which of the following is the pH of 0.08 mol dm-3 Ba(OH)2? 

(Kw = 1.00 x 10-14 mol2 dm-6 at 25 ℃)

  • 14.80

  • 13.00

  • 13.20

  • 12.90

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3
1 mark

What is the pH of ethanoic acid with a concentration of 0.75 mol dm-3 at 25 oC? 

(Ka of ethanoic acid at 25 ℃ = 1.78 x 10-5 mol dm-3)

  • 0.12

  • 2.44

  • 4.62

  • 9.72

Choose your answer

4
1 mark

The pH range of four different indicators are:

Indicator

pH range for colour change

Methyl orange

3.2 – 4.4

Phenolphthalein

8.2 – 10.0

Thymol blue

1.2 – 2.8

Bromothymol blue

6.0 – 7.6

Which indicator would be suitable for a titration involving hydrochloric acid and ammonia?

  • Methyl orange

  • Phenolphthalein

  • Thymol blue

  • Bromothymol blue

Choose your answer

5
1 mark

What is the pH of a buffer solution containing 0.0413 moles of ethanoic acid and 0.0398 moles of sodium ethanoate in 500 cm3 at 25 oC? 

(Ka of ethanoic acid at 25 ℃ = 1.78 x 10-5 mol dm-3)

  • 1.08

  • 2.91

  • 4.73

  • 4.77

Choose your answer

6
1 mark

Which statement is correct for the following equation?

NH4+ + OH– ⇌ H2O + NH3

  • NH4+ is the base

  • H2O is the conjugate acid

  • NH3 is the conjugate acid

  • OH– is the acid

Choose your answer

7
1 mark

25.00 cm3 of 0.25 mol dm−3 nitric acid is added to 12.50 cm3 of 0.10 mol dm−3 sodium hydroxide.

What is the pH of the resulting mixture?

  • 0.60

  • 0.88

  • 2.30

  • 13.00

Choose your answer

8
1 mark

Nitrous acid, HNO2, is a weak acid with an acid dissociation constant, Ka, of 4.7 x 10-4 mol dm-3 at 25 oC.

What is the pH of a 0.040 mol dm-3 solution of nitrous acid?

  • 1.40

  • 2.36

  • 3.33

  • 4.73

Choose your answer

1
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1 mark

A buffer solution is prepared by mixing 200 cm3 of 2.00 mol dm–3 propanoic acid, CH3CH2COOH, with 600 cm3 of 1.00 mol dm–3 sodium propanoate, CH3CH2COONa.

Ka for CH3CH2COOH = 1.32 × 10–5 mol dm–3

What is the pH of the buffer solution?

  • 4.58

  • 4.70

  • 4.88

  • 5.06

Choose your answer

2
1 mark

Which of the following statements is/are correct?

  1. The initial concentration of H2CO3 can be used to calculate Ka.

  2. When calculating the Ka of H2CO3, [H+] equals [A-] at equilibrium.

  3. The value of Ka is independent of temperature. 

  • 1, 2 and 3

  • Only 1 and 2

  • Only 2 and 3

  • Only 1

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3
1 mark

X g of potassium ethanoate was added to ethanoic acid to form a buffer solution with a final volume of 250 cm3 and a pH of 4.70. The final concentration of ethanoic acid was 0.40 mol dm-3.

Ka for ethanoic acid is 1.8 x 10-5 mol dm-3.

What was the mass of potassium ethanoate added?

  • 5.41 g

  • 8.85 g

  • 41.6 g

  • 68.0 g

Choose your answer

4
1 mark

A student has the following solutions:

Solution

Potassium hydroxide, KOH (aq)

0.72 mol dm-3

Sodium hydroxide, NaH (aq)

pH = 13.4

Hydrogen cyanide, HCN (aq)

pH = 4.20

Ka = 6.2 x 10-10 mol dm-3

Kw = 1.00 x 10-14 mol2 dm-6.

Which row correctly describes the pH of potassium hydroxide, the concentration of sodium hydroxide (mol dm-3) and the concentration of hydrogen cyanide (mol dm-3)?

pH of KOH

[NaOH]

[HCN]

A

13.90

0.60

6.42

B

13.86

0.60

4.05

C

13.86

0.25

6.42

D

13.80

0.25

6.42

    Choose your answer

    5
    1 mark

    What is the pH of a buffer solution formed when 2.5 g of NaOH is added to 1.0 dm3 of 0.15 mol dm-3 solution of ethanoic acid?

    Ka for ethanoic acid is 1.74 x 10-5 mol dm-3.

    • 4.19

    • 4.61

    • 5.58

    • 5.96

    Choose your answer