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Further Acids & Bases Calculations (AQA A Level Chemistry)

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11 mark

The correct expression for the acid dissociation constant, Ka, for the dissociation of benzoic acid, C6H5COOH is

  • Ka =  fraction numerator left square bracket straight C subscript 6 straight H subscript 5 COOH right square bracket over denominator left square bracket straight H to the power of plus right square bracket left square bracket straight C subscript 6 straight H subscript 5 COO to the power of minus right square bracket end fraction

  • Ka = fraction numerator left square bracket straight H to the power of plus right square bracket left square bracket straight C subscript 6 straight H subscript 5 COO to the power of minus right square bracket over denominator left square bracket straight C subscript 6 straight H subscript 5 COOH right square bracket end fraction

  • Ka = [H+][C6H5COO-]

  • Ka = [C6H5COOH]

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21 mark

The pH curve represents which type of titration? 

further-c

  • 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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31 mark

What is the pH of a buffer solution containing 0.80 mol dm-3 solution of ethanoic acid, CH3COOH, and 0.80 mol dm-3 sodium ethanoate, CH3COONa at 25 ℃?

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

  • 0.10

  • 5.75

  • 3.75

  • 4.75

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41 mark

Which of the following statements about pKa is correct?

  • The higher the value, the stronger the acid

  • The lower the value. the stronger the acid

  • pKa = 10-Ka

  • Ka = -log pKa

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51 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

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11 mark

The pH of ethanoic acid with a concentration of 0.75 mol dm-3 at 25 ℃ is:

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

  • 4.62

  • 0.12

  • 9.72

  • 2.44

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21 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

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31 mark

The pH curve below represents a weak acid, strong base titration 

5-6-chem--q3

The Ka of the weak acid is:

  • 4.90

  • 8.72

  • 1.26 x 10-5

  • 1.91 x 10-9

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41 mark

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 ℃ is: 

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

  • 1.08

  • 4.73

  • 2.91

  • 4.74

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51 mark

The table shows the pKa values for three acids

              Acid               

     pKa         

Benzoic acid

       4.2

Nitrous acid

       3.1

Lactic acid 

       3.4

Which of the following statements is correct? 

  • The Ka value for lactic acid at 25 ℃ is 3.98 x 10-5 mol dm-3

  • The Ka value for nitrous acid at 25 ℃ is 7.94 x 10-4 mol dm-3

  • Nitrous acid is the weakest acid

  • Benzoic acid is the strongest acid

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11 mark

In a titration. the equivalence point is reached when 23.70 cm3 of 0.02 mol dm-3 potassium hydroxide reacts with 0.03 mol dm-3 of sulfuric acid. What volume of acid is needed?

  • 29.80 cm3

  • 15.80 cm3

  • 7.90 cm3

  • 1.26 x 105 cm3

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21 mark

What is the pH of the acidic solution when 0.05 moles of benzoic acid is dissolved in 2 dm3 of water?

(pKa of benzoic acid at 25 oC = 4.20)

  • 5.80

  • 3.70

  • 2.90

  • 2.10

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31 mark

An acid base titration is represented by part of the pH curve shown below.

Graph showing pH change with NaOH volume, starting at pH 3, rising steadily until 10 cm³, then sharply increasing to pH 7 at 14 cm³.

What is the Ka of the acid?

  • 5.50 mol dm-3

  • 3.50 mol dm-3

  • 3.16 x 10-4 mol dm-3

  • 3.16 x 10-6 mol dm-3

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41 mark

The correct solution to form a buffer with a pH of 4.77 is:

(Ka of ethanoic acid at 25 oC = 1.7 x 10-5 mol dm-3)

(Ka of methanoic acid at 25 oC = 1.8 x 10-4 mol dm-3)

  • 0.100 mol dm-3 ethanoic acid 500 cm3 with 0.100 mol dm sodium hydroxide 250 cm3

  • 0.100 mol dm-3 ethanoic acid 500 cm3 with 0.200 mol dm-3 sodium hydroxide 250 cm3

  • 0.100 mol dm-3 methanoic acid 500 cm3 with 0.100 mol dm-3 sodium hydroxide 250 cm3

  • 0.100 mol dm-3 methanoic acid 500 cm3 with 0.200 mol dm-3 sodium hydroxide 250 cm3

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51 mark

What is the pH of the buffer solution made using 0.30 mol dm-3 ethanoic acid, CH3COOH (aq), and 0.20 mol dm-3 sodium ethanoate after 20.00 cm3 of 0.20 mol dm-3 hydrochloric acid, HCl (aq), is added?

(Ka of ethanoic acid at 25 oC = 1.7 x 10-5 mol dm-3 )

  • 4.94

  • 4.77

  • 2.40

  • 4.58

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61 mark

Some information about a solution of hydrofluoric acid, HF (aq), is given in the following table.

pH value

2.01

pKa

3.3

The concentration of the acid is?

  • 0.19 mol dm-3

  • 9.77 x 10 -3mol dm-3

  • 0.51 mol dm-3

  • 5.01 x 10-4

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