Edexcel A Level Chemistry

Topic Questions

A LevelChemistryEdexcelTopic Questions5. Advanced Physical Chemistry (A Level Only)5.2 Acid-base Equilibria

5.2 Acid-base Equilibria

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1a1 mark

This question is about acids and buffer solutions.

Ethanoic acid, CH3COOH, is a monobasic acid.

CH3COOH + H2O ⇌ CH3COO–+ H3O+

Give a reason why only the proton from the carboxylic acid group, and not from the methyl group, is donated to a water molecule.

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1b1 mark

The reaction of ammonia with water can be represented by

NH3 + H2O ⇌ Nbegin mathsize 16px style H subscript 4 to the power of plus sign end style+ OH–

Which is the acid-conjugate base pair?

      Acid Conjugate base
  ☐ A NH3 OH–
  ☐ B NH3 Nstraight H subscript 4 superscript plus
  ☐ C H2O OH–
  ☐ D H2O Nstraight H subscript 4 superscript plus

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1c
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A commercial nitric acid solution, HNO3 (aq), has a concentration of 15.9 mol dm–3.

A 15.0 cm3 sample was made up to 100 cm3 by adding deionised water.

Calculate the pH of this diluted solution.

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1d
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Propanoic acid is a weak acid.

i)
Calculate the pH of a 0.100 mol dm–3 solution of propanoic acid at 298 K.
Give your answer to an appropriate number of significant figures.

[Ka = 1.35×10–5 mol dm–3 at 298 K]

(3)

ii)
State two assumptions that you made in the calculation in d(i).
(2)

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1e
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A buffer solution was made using 20.0 cm3 of a butanoic acid solution, of concentration 0.100 mol dm–3 and 30.0 cm3of sodium butanoate solution, of concentration 0.305 mol dm–3.

[Ka = 1.52×10–5 mol dm–3 at 298 K]

i)
Calculate the pH of this buffer solution at 298 K.

(4)

ii)
Explain why the pH of the buffer solution hardly changes when a few drops of sodium hydroxide solution are added to it.
Include an equation or equations in your answer.
Use C3H7COOH as the formula for butanoic acid.

(2)

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2a1 mark

Boric acid, H3BO3, is a weak acid with antiseptic properties.

Boric acid can be prepared by reacting borax, Na2B4O7.10H2O, with hydrochloric acid.


Write the equation for this reaction. State symbols are not required.

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2b3 marks

The formula of boric acid can also be written as B(OH)3.

i)

Complete the dot-and-cross diagram of a molecule of boric acid.
Show the outer shell electrons only.

Use dots (•) for the hydrogen electrons, crosses (×) for the oxygen electrons and triangles ( Δ ) for the boron electrons.

(2)

q7bi-9cho-al-1-june-2018-qp-edexcel-a-level-chem

ii)
What are the O—B—O and B—O—H bond angles in a molecule of boric acid?
(1)
      O—B—O bond angle  B—O—H bond angle
  ☐ A 109.5° 104.5°
  ☐ B 109.5° 180°
  ☐ C 120° 104.5°
  ☐ D 120° 180°

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2c1 mark

Boric acid is a solid with melting temperature 171°C.

What are the strongest interactions between the molecules in solid boric acid?

  ☐ A covalent bonds
  ☐ B hydrogen bonds
  ☐ C ionic bonds
  ☐ D London forces

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2d
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In aqueous solution, boric acid dissociates into ions in three stages.

The equation for the first dissociation is

H3BO3(aq) ⇌ H+(aq) + H2BO3-(aq)

pKa for this dissociation is 9.24

i)
Calculate the pH of a 0.0500 mol dm−3 solution of boric acid from the pKa value for the first dissociation.

(3)

ii)
State any assumptions you made in your calculation in (d)(i).
(2)

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2e1 mark

Boric acid can undergo further dissociation.
Which is the conjugate acid of the HBO32– ion?

  ☐ A BO33–
  ☐ B H2BO3–
  ☐ C H3BO3
  ☐ D H3O+

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3a
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This question is about weak acids.

A weak acid, HX, has a Ka value of 5.25 × 10−5 mol dm−3. A solution was formed by mixing 10.5 cm3 of 0.800 mol dm−3 dilute sodium hydroxide with 25.0 cm3 of 0.920 mol dm−3 HX(aq).
Calculate the pH of the solution formed, showing all your working.

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3b6 marks
i)
Propanoic acid, CH3CH2COOH, is a weak acid.
On the grid below, sketch the change in pH during the addition of 50.0 cm3 of 0.100 mol dm−3 sodium hydroxide solution to 25.0 cm3 of 0.100 mol dm−3 propanoic acid solution.

(4)

q7bi-9cho-al-3-june-2018-qp-edexcel-a-level-chem

ii)
Explain how you would use the graph in (b)(i) to obtain the value of the acid dissociation constant, Ka , for propanoic acid.
You are not expected to calculate this value.

(2)

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4a1 mark

This is a question about buffer solutions.

A buffer solution always

  ☐ A keeps the pH less than 7.
  ☐ B contains equimolar amounts of acid and its conjugate base.
  ☐ C keeps the pH constant if small quantities of acid or base are added.
  ☐ D resists changes in pH if small quantities of acid or base are added.

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4b
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A buffer solution with a pH of 3.90 is required.
Calculate the mass, in grams, of sodium ethanoate that should be added to 50.0 cm3 of an ethanoic acid solution of concentration 0.800 mol dm–3 to form this buffer solution.

Give your answer to an appropriate number of significant figures.

[Ka for ethanoic acid = 1.74 × 10–5 mol dm–3]

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4c3 marks

One of the systems controlling the pH of blood is the carbonic acid-hydrogencarbonate buffer system.

H2CO3 rightwards harpoon over leftwards harpoon H+ + HCO3–

Explain how this buffer system helps to control the pH of blood when extra carbon dioxide is present due to strenuous exercise.

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4d6 marks

A weak acid-strong base titration curve can be used to demonstrate buffer action.

i)
Draw a titration curve for the addition of 100 cm3 of sodium hydroxide solution of concentration 0.100 mol dm–3 to 40.0 cm3 of propanoic acid solution of concentration 0.100 mol dm–3 which has a pH of 3.0.

Show the part of the curve that demonstrates buffer action.

(4)

q9d-9cho-al-1-june-2019-qp-edexcel-a-level-chem

ii)
Describe, without calculation, how you would use your curve to determine the value of Ka for propanoic acid.

(2)

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5a1 mark

This question is about acids and bases.

State what is meant by a Brønsted-Lowry acid.

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5b1 mark

Identify the acid-base conjugate pairs in this reaction.

CH3COOH + HCOOH → CH3COOH2+ + HCOO–

................... + ................... → .................... + ....................

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5c1 mark

Write the expression that defines the pH of a solution.

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5d
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Calculate the concentration of hydrogen ions, in mol dm-3, in a solution with a pH of 2.76

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5e
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Explain why the pH of a 1 × 10-8 mol dm-3 solution of nitric acid, HNO3, is not 8.
[Ionic product of water, Kw = 1.00 × 10-14 mol2 dm-6]

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1a1 mark

This question is about acids and bases.

What is the order of decreasing pH for 0.100 mol dm−3 solutions of these three acids?

  ☐ A CH3COOH > CH2ClCOOH > HCl
  ☐ B HCl > CH3COOH > CH2ClCOOH
  ☐ C CH2ClCOOH > CH3COOH >HCl
  ☐ D HCl > CH2ClCOOH > CH3COOH

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1b
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A solution of methanoic acid, HCOOH, has a concentration of 0.240 mol dm−3 and a pH of 2.20.

Calculate the value of pKa for methanoic acid.

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1c1 mark

Which of these mixtures would form a buffer solution with a pH below 7?

  ☐ A NaOH (aq) and excess HCl (aq)
  ☐ B NaOH (aq) and excess CH3COOH (aq)
  ☐ C excess NaOH (aq) and HCl (aq)
  ☐ D excess NaOH (aq) and CH3COOH (aq)

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1d1 mark

Bromothymol blue, methyl orange and phenolphthalein are indicators used in titrations.

Which, if any, of these indicators could be used for a titration of ammonia, NH3 (aq), with ethanoic acid, CH3COOH (aq)?

  ☐ A bromothymol blue
  ☐ B methyl orange
  ☐ C phenolphthalein
  ☐ D none of these three indicators

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2a4 marks

This is a question about water.

Water might be expected to have a lower boiling temperature than hydrogen sulfide but it actually has a higher boiling temperature.

Comment on this statement by referring to the intermolecular forces in both these substances.

A detailed description of how the intermolecular forces arise is not required.

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2b4 marks

Explain why both water and carbon dioxide molecules have polar bonds but only water is a polar molecule.

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2c2 marks

Pure water ionises to form H3O+  and OH- ions, although only to a very small extent.
Draw the dot-and-cross diagrams of these ions.
Use dots (•) for the hydrogen electrons and crosses (x) for the oxygen electrons.

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2d
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An equation for the ionisation of water is

H2O (I) rightwards harpoon over leftwards harpoon H+ (aq) + OH- (aq)

The expression for the ionic product of water is

Kw = [H+(aq)][OH-(aq)]


The value of Kw at 310 K is 2.40 × 10–14 mol2 dm–6

i)
Calculate the pH of water at 310 K.

Give your answer to two decimal places.

(2)

ii)
Predict, with a reason, whether water is acidic, alkaline or neutral at 310 K.

(2)

iii)
Predict, with a reason, the sign of the enthalpy change for the ionisation of water.

(1)

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