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A Level H1 Chemistry Acids Bases Salts Quiz

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Questions

A-Level Chemistry H1 Quiz - Acids Bases Salts

Name: _______________________ Class: _______________________

Date: _______________________ Score: ________ / 50

Duration: 60 minutes

Instructions:

Answer all questions in the spaces provided.
Show all working for calculation questions. Answers without working may not receive full marks.
Use appropriate units and significant figures where applicable.
A Periodic Table and data booklet are provided separately.

Section A: Multiple Choice (Questions 1–5)

Choose the most appropriate answer for each question. Each question carries 1 mark.

1. Which of the following best defines a Brønsted–Lowry acid?

A. A substance that donates a proton ( $H^+$ )
B. A substance that accepts a proton ( $H^+$ )
C. A substance that produces $H^+$ ions in aqueous solution
D. A substance with a pH greater than 7

2. The pH of a $0.05 \text{ mol dm}^{-3}$ solution of a strong monoprotic acid is closest to:

A. 0.3
B. 1.0
C. 1.3
D. 2.0

3. Which of the following salts will produce a solution with pH > 7 when dissolved in water?

A. Ammonium chloride, $NH_4Cl$
B. Sodium chloride, $NaCl$
C. Sodium ethanoate, $CH_3COONa$
D. Potassium nitrate, $KNO_3$

4. A buffer solution is prepared by mixing $25 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3}$ $CH_3COOH$ with $25 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3}$ $CH_3COONa$ . Which statement about this buffer is correct?

A. The pH of the buffer is exactly 7.
B. The buffer resists pH change only when acid is added.
C. The buffer contains a weak acid and its conjugate base.
D. Adding a small amount of $NaOH$ will not change the pH at all.

5. The ionic product of water, $K_w$ , at $25^\circ\text{C}$ is $1.0 \times 10^{-14} \text{ mol}^2 \text{ dm}^{-6}$ . What is the pH of pure water at $50^\circ\text{C}$ if $K_w = 5.5 \times 10^{-14} \text{ mol}^2 \text{ dm}^{-6}$ at this temperature?

A. 6.63
B. 7.00
C. 7.37
D. 13.26

Section B: Structured Questions (Questions 6–15)

Answer all questions. Show all working where applicable.

6. Define the following terms:

(a) A weak acid. [2]

(b) A buffer solution. [2]

7. Ethanoic acid ( $CH_3COOH$ ) is a weak acid with $K_a = 1.74 \times 10^{-5} \text{ mol dm}^{-3}$ at $25^\circ\text{C}$ .

(a) Write an expression for the acid dissociation constant, $K_a$ , of ethanoic acid. [1]

(b) Calculate the pH of a $0.20 \text{ mol dm}^{-3}$ solution of ethanoic acid at $25^\circ\text{C}$ . Give your answer to 2 decimal places. [3]

8. A student titrates $25.0 \text{ cm}^3$ of $0.100 \text{ mol dm}^{-3}$ hydrochloric acid with $0.100 \text{ mol dm}^{-3}$ sodium hydroxide.

(a) Write the balanced equation for the reaction. [1]

(b) Calculate the volume of sodium hydroxide required to reach the equivalence point. [2]

9. A buffer solution is made by mixing $50.0 \text{ cm}^3$ of $0.200 \text{ mol dm}^{-3}$ ethanoic acid with $50.0 \text{ cm}^3$ of $0.100 \text{ mol dm}^{-3}$ sodium ethanoate.

(a) Calculate the concentration of ethanoic acid and sodium ethanoate in the buffer after mixing. [2]

(b) Using your answer in (a), calculate the pH of this buffer. ( $K_a$ of ethanoic acid $= 1.74 \times 10^{-5} \text{ mol dm}^{-3}$ ) [3]

10. Explain why the pH of a buffer solution remains approximately constant when a small amount of dilute hydrochloric acid is added. Your answer should refer to the equilibrium involved. [3]

11. The titration curve below shows the pH change when $25.0 \text{ cm}^3$ of a weak acid is titrated with $0.100 \text{ mol dm}^{-3}$ sodium hydroxide.

<image_placeholder> id: Q11-fig1 type: graph linked_question: Q11 description: Titration curve showing pH (y-axis, 0–14) against volume of NaOH added in cm³ (x-axis, 0–50). Curve starts at pH ~3, rises gradually, then has a steep vertical rise between 20–30 cm³, passing through pH 7 at ~25 cm³ (equivalence point), then levels off around pH 11–12. The buffer region (flat portion before the steep rise) is visible between ~5–20 cm³. The half-equivalence point (where pH = pKa) is marked at ~12.5 cm³, pH ~4.76. labels: y-axis: "pH", x-axis: "Volume of NaOH added / cm³", equivalence point at 25 cm³, half-equivalence point at 12.5 cm³, buffer region shaded between 5–20 cm³ values: initial pH ≈ 3, equivalence point at 25 cm³ NaOH, pH at equivalence ≈ 8.7, half-equivalence pH ≈ 4.76, steep rise from 20–30 cm³ must_show: y-axis labelled "pH" with scale 0–14, x-axis labelled "Volume of NaOH added / cm³" with scale 0–50, equivalence point clearly marked at 25 cm³, buffer region visible as a relatively flat portion before the steep rise, curve starting at low pH (~3) and ending at high pH (~11–12) </image_placeholder>

(a) From the curve, estimate the pH at the equivalence point. [1]

(b) Explain why the pH at the equivalence point is greater than 7. [2]

(c) State the volume of sodium hydroxide at which the half-equivalence point occurs. What is the significance of this point? [2]

12. A solution of ammonia ( $NH_3$ ) has a concentration of $0.10 \text{ mol dm}^{-3}$ . The $K_b$ of ammonia is $1.8 \times 10^{-5} \text{ mol dm}^{-3}$ .

(a) Write an expression for the base dissociation constant, $K_b$ , of ammonia. [1]

(b) Calculate the pH of this ammonia solution. Give your answer to 2 decimal places. [4]

13. Describe an experiment to determine the concentration of a solution of hydrochloric acid using a standard solution of sodium hydroxide. Include the apparatus, procedure, and how you would identify the end point. [4]

14. Sodium carbonate ( $Na_2CO_3$ ) is a salt that dissolves in water to form a basic solution.

(a) Write the equation for the hydrolysis of the carbonate ion ( $CO_3^{2-}$ ) in water. [1]

(b) Explain why the solution is basic, with reference to the relative strengths of the acid and base involved. [2]

15. A solution contains a mixture of $0.10 \text{ mol dm}^{-3}$ $HCl$ and $0.10 \text{ mol dm}^{-3}$ $CH_3COOH$ ( $K_a = 1.74 \times 10^{-5} \text{ mol dm}^{-3}$ ).

(a) Explain why the pH of this mixture is determined almost entirely by the $HCl$ and not by the ethanoic acid. [2]

(b) Calculate the pH of this mixture. [2]

Section C: Application & Data Interpretation (Questions 16–20)

Answer all questions. Show all working where applicable.

16. A student investigates the effectiveness of two buffer solutions. Buffer A contains $0.10 \text{ mol dm}^{-3}$ $CH_3COOH$ and $0.10 \text{ mol dm}^{-3}$ $CH_3COONa$ . Buffer B contains $0.20 \text{ mol dm}^{-3}$ $CH_3COOH$ and $0.20 \text{ mol dm}^{-3}$ $CH_3COONa$ .

The student adds $5.0 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3}$ $HCl$ to $100 \text{ cm}^3$ of each buffer and measures the pH change.

(a) Without calculation, predict which buffer (A or B) will show the smaller pH change. Explain your reasoning. [2]

(b) Calculate the pH of Buffer A before and after the addition of $HCl$ . ( $K_a = 1.74 \times 10^{-5} \text{ mol dm}^{-3}$ ) [5]

17. The table below shows the $K_a$ values of three weak acids at $25^\circ\text{C}$ .

Acid	Formula	$K_a$ / $\text{mol dm}^{-3}$
Methanoic acid	$HCOOH$	$1.6 \times 10^{-4}$
Ethanoic acid	$CH_3COOH$	$1.74 \times 10^{-5}$
Carbonic acid	$H_2CO_3$	$4.3 \times 10^{-7}$

(a) Arrange the three acids in order of decreasing acid strength. [1]

(b) Calculate the pH of a $0.050 \text{ mol dm}^{-3}$ solution of methanoic acid. Give your answer to 2 decimal places. [3]

18. A solution of sodium hydroxide has a pH of 13.0 at $25^\circ\text{C}$ .

(a) Calculate the concentration of $OH^-$ ions in this solution. [2]

(b) This $NaOH$ solution is used to titrate $25.0 \text{ cm}^3$ of a solution of sulfuric acid ( $H_2SO_4$ ). The volume of $NaOH$ required to reach the equivalence point is $35.0 \text{ cm}^3$ . Calculate the concentration of the sulfuric acid. [4]

19. A student prepares a buffer by dissolving $8.20 \text{ g}$ of sodium ethanoate ( $CH_3COONa$ , $M_r = 82.0$ ) in $250 \text{ cm}^3$ of $0.100 \text{ mol dm}^{-3}$ ethanoic acid.

(a) Calculate the number of moles of sodium ethanoate dissolved. [1]

(b) Calculate the pH of the resulting buffer solution. ( $K_a = 1.74 \times 10^{-5} \text{ mol dm}^{-3}$ ) [3]

20. The indicator methyl orange has a $pK_{In}$ of 3.7. It is red in acidic solution and yellow in alkaline solution.

(a) Explain why methyl orange is not suitable for the titration of a weak acid with a strong base. [2]

(b) Suggest a more suitable indicator for this titration and explain your choice. [2]

End of Quiz

Total Marks: 50

Answers

A-Level Chemistry H1 Quiz - Acids Bases Salts

Answer Key & Teaching Notes

Section A: Multiple Choice

1. Answer: A

Explanation: A Brønsted–Lowry acid is defined as a proton ( $H^+$ ) donor. Option C describes an Arrhenius acid, which is a related but narrower definition. Option B describes a Brønsted–Lowry base, and Option D describes an alkaline substance. At A-Level, the Brønsted–Lowry definition is the one most commonly tested.

Common mistake: Students confuse Arrhenius and Brønsted–Lowry definitions. While all Brønsted–Lowry acids produce $H^+$ in water, the defining feature is proton donation, not merely producing $H^+$ ions.

2. Answer: C

Explanation: For a strong monoprotic acid, $[H^+] = 0.05 \text{ mol dm}^{-3}$ (complete dissociation). Therefore:

$\text{pH} = -\log_{10}[H^+] = -\log_{10}(0.05) = -\log_{10}(5 \times 10^{-2}) = 2 - \log_{10}5 = 2 - 0.70 = 1.30$

Common mistake: Students may incorrectly calculate $\log_{10}(0.05)$ as 0.3 (confusing $\log_{10}$ with $\ln$ ), or forget that strong acids fully dissociate so $[H^+]$ equals the acid concentration.

3. Answer: C

Explanation: Sodium ethanoate is the salt of a weak acid (ethanoic acid) and a strong base (sodium hydroxide). The ethanoate ion ( $CH_3COO^-$ ) undergoes hydrolysis in water:

$CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-$

This produces $OH^-$ ions, making the solution basic (pH > 7).

$NH_4Cl$ : salt of weak base + strong acid → acidic (pH < 7)
$NaCl$ : salt of strong acid + strong base → neutral (pH = 7)
$KNO_3$ : salt of strong acid + strong base → neutral (pH = 7)

Teaching note: The rule is: "the stronger wins." If the parent acid is weaker than the parent base is strong, the solution is basic, and vice versa.

4. Answer: C

Explanation: A buffer solution consists of a weak acid and its conjugate base (or a weak base and its conjugate acid). Here, $CH_3COOH$ is the weak acid and $CH_3COO^-$ (from $CH_3COONa$ ) is the conjugate base.

A is incorrect: the pH of an ethanoate buffer is approximately 4.76 (equal to $pK_a$ ), not 7.
B is incorrect: buffers resist pH change when either acid OR base is added.
D is incorrect: the pH changes slightly but not dramatically. The buffer minimises the change; it does not eliminate it entirely.

5. Answer: A

Explanation: In pure water, $[H^+] = [OH^-]$ , so:

$K_w = [H^+][OH^-] = [H^+]^2$

$[H^+] = \sqrt{K_w} = \sqrt{5.5 \times 10^{-14}} = 2.345 \times 10^{-7} \text{ mol dm}^{-3}$

$\text{pH} = -\log_{10}(2.345 \times 10^{-7}) = 7 - \log_{10}(2.345) = 7 - 0.37 = 6.63$

Teaching note: Pure water is always neutral ( $[H^+] = [OH^-]$ ), but the pH is only 7.00 at $25^\circ\text{C}$ where $K_w = 1.0 \times 10^{-14}$ . At higher temperatures, $K_w$ increases, so pH of pure water decreases below 7 — but it is still neutral.

Common mistake: Students think pH 7 always means neutral. Neutrality means $[H^+] = [OH^-]$ , which depends on temperature through $K_w$ .

Section B: Structured Questions

6. (a) [2 marks]

A weak acid is an acid that partially dissociates (ionises) in aqueous solution, establishing an equilibrium between the undissociated acid and its ions.

Mark allocation:

1 mark for "partially dissociates" or "partially ionises"
1 mark for reference to equilibrium / not fully dissociated

Common mistake: Writing "dilute acid" instead of "weak acid." Dilute refers to concentration; weak refers to the degree of dissociation. A concentrated weak acid is still weak.

(b) [2 marks]

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added (or when it is diluted). It typically contains a weak acid and its conjugate base (or a weak base and its conjugate acid).

Mark allocation:

1 mark for "resists pH change" / "minimises pH change"
1 mark for "on addition of small amounts of acid or base" AND reference to weak acid-conjugate base (or weak base-conjugate acid)

7. (a) [1 mark]

$K_a = \frac{[CH_3COO^-][H^+]}{[CH_3COOH]}$

Note: Water is omitted from the expression because it is the solvent and its concentration is essentially constant.

(b) [3 marks]

For a weak acid, assuming $x$ is small compared to initial concentration:

$K_a = \frac{[H^+]^2}{[CH_3COOH]_{\text{initial}}}$

$[H^+]^2 = K_a \times [CH_3COOH] = 1.74 \times 10^{-5} \times 0.20 = 3.48 \times 10^{-6}$

$[H^+] = \sqrt{3.48 \times 10^{-6}} = 1.865 \times 10^{-3} \text{ mol dm}^{-3}$

$\text{pH} = -\log_{10}(1.865 \times 10^{-3}) = 2.73$

Mark allocation:

1 mark for correct $K_a$ expression or substitution
1 mark for correct $[H^+]$ calculation
1 mark for correct pH to 2 d.p.

Common mistake: Forgetting to take the square root, or using $[H^+] = K_a \times [HA]$ instead of $[H^+] = \sqrt{K_a \times [HA]}$ . The approximation $[H^+] = \sqrt{K_a \cdot C}$ is valid when the degree of dissociation is small (< 5%).

8. (a) [1 mark]

$HCl + NaOH \rightarrow NaCl + H_2O$

or in ionic form: $H^+ + OH^- \rightarrow H_2O$

(b) [2 marks]

$n(HCl) = 0.100 \times \frac{25.0}{1000} = 2.50 \times 10^{-3} \text{ mol}$

Since the mole ratio is 1:1:

$n(NaOH) = 2.50 \times 10^{-3} \text{ mol}$

$V(NaOH) = \frac{2.50 \times 10^{-3}}{0.100} = 2.50 \times 10^{-2} \text{ dm}^3 = 25.0 \text{ cm}^3$

Mark allocation:

1 mark for moles of HCl calculated correctly
1 mark for correct volume of NaOH

(c) [2 marks]

The pH at the equivalence point is 7.

Explanation: $HCl$ is a strong acid and $NaOH$ is a strong base. The salt formed ( $NaCl$ ) is derived from a strong acid and strong base, so it does not hydrolyse. The solution is neutral at pH 7.

Mark allocation:

1 mark for stating pH = 7
1 mark for correct explanation (strong acid + strong base → neutral salt)

9. (a) [2 marks]

Total volume after mixing $= 50.0 + 50.0 = 100.0 \text{ cm}^3$

$[CH_3COOH] = \frac{0.200 \times 50.0}{100.0} = 0.100 \text{ mol dm}^{-3}$

$[CH_3COONa] = \frac{0.100 \times 50.0}{100.0} = 0.050 \text{ mol dm}^{-3}$

Mark allocation:

1 mark for each correct concentration

(b) [3 marks]

Using the Henderson–Hasselbalch equation:

$\text{pH} = pK_a + \log_{10}\frac{[\text{salt}]}{[\text{acid}]}$

$pK_a = -\log_{10}(1.74 \times 10^{-5}) = 4.76$

$\text{pH} = 4.76 + \log_{10}\left(\frac{0.050}{0.100}\right) = 4.76 + \log_{10}(0.50) = 4.76 - 0.30 = 4.46$

Alternative method using $K_a$ expression:

$K_a = \frac{[CH_3COO^-][H^+]}{[CH_3COOH]}$

$1.74 \times 10^{-5} = \frac{0.050 \times [H^+]}{0.100}$

$[H^+] = \frac{1.74 \times 10^{-5} \times 0.100}{0.050} = 3.48 \times 10^{-5} \text{ mol dm}^{-3}$

$\text{pH} = -\log_{10}(3.48 \times 10^{-5}) = 4.46$

Mark allocation:

1 mark for correct $pK_a$ or $K_a$ expression
1 mark for correct substitution
1 mark for correct final pH

10. [3 marks]

When a small amount of $HCl$ is added to a buffer containing $CH_3COOH$ and $CH_3COO^-$ :

The added $H^+$ ions react with the conjugate base ( $CH_3COO^-$ ):

$CH_3COO^- + H^+ \rightarrow CH_3COOH$

This shifts the equilibrium $CH_3COOH \rightleftharpoons CH_3COO^- + H^+$ to the left (Le Chatelier's principle). The added $H^+$ is consumed by the $CH_3COO^-$ ions, converting them into $CH_3COOH$ . Since the ratio $\frac{[CH_3COO^-]}{[CH_3COOH]}$ changes only slightly, the pH remains approximately constant.

Mark allocation:

1 mark for stating that added $H^+$ reacts with the conjugate base ( $CH_3COO^-$ )
1 mark for reference to equilibrium shifting left / Le Chatelier's principle
1 mark for explaining that the ratio changes minimally, so pH is approximately constant

11. (a) [1 mark]

From the titration curve, the pH at the equivalence point is approximately 8.7 (accept 8.5–9.0).

(b) [2 marks]

The pH at the equivalence point is greater than 7 because the salt formed is the salt of a weak acid and a strong base (e.g., sodium ethanoate). The conjugate base of the weak acid undergoes hydrolysis:

$CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-$

This produces $OH^-$ ions, making the solution slightly alkaline.

Mark allocation:

1 mark for identifying the salt as being from a weak acid + strong base
1 mark for explaining hydrolysis producing $OH^-$ ions

(c) [2 marks]

The half-equivalence point occurs at 12.5 cm³ (half the volume at the equivalence point).

Significance: At the half-equivalence point, exactly half the weak acid has been neutralised, so $[\text{acid}] = [\text{conjugate base}]$ . From the Henderson–Hasselbalch equation:

$\text{pH} = pK_a + \log_{10}(1) = pK_a$

Therefore, pH = $pK_a$ at this point, allowing the $K_a$ of the weak acid to be determined directly from the titration curve.

Mark allocation:

1 mark for correct volume (12.5 cm³)
1 mark for stating that pH = $pK_a$ at this point

12. (a) [1 mark]

$K_b = \frac{[NH_4^+][OH^-]}{[NH_3]}$

(b) [4 marks]

$K_b = \frac{[OH^-]^2}{[NH_3]_{\text{initial}}}$

$[OH^-]^2 = K_b \times [NH_3] = 1.8 \times 10^{-5} \times 0.10 = 1.8 \times 10^{-6}$

$[OH^-] = \sqrt{1.8 \times 10^{-6}} = 1.342 \times 10^{-3} \text{ mol dm}^{-3}$

$\text{pOH} = -\log_{10}(1.342 \times 10^{-3}) = 2.87$

$\text{pH} = 14.00 - 2.87 = 11.13$

Alternative (direct pH calculation):

$[H^+] = \frac{K_w}{[OH^-]} = \frac{1.0 \times 10^{-14}}{1.342 \times 10^{-3}} = 7.45 \times 10^{-12}$

$\text{pH} = -\log_{10}(7.45 \times 10^{-12}) = 11.13$

Mark allocation:

1 mark for correct $K_b$ expression / substitution
1 mark for correct $[OH^-]$
1 mark for correct pOH
1 mark for correct final pH to 2 d.p.

13. [4 marks]

Apparatus: Burette, pipette ( $25.0 \text{ cm}^3$ ), pipette filler, conical flask, white tile, standard $NaOH$ solution, $HCl$ solution of unknown concentration, indicator (e.g., phenolphthalein or methyl orange).

Procedure:

Use a pipette to measure $25.0 \text{ cm}^3$ of the $HCl$ solution into a conical flask.
Add a few drops of indicator.
Fill the burette with the standard $NaOH$ solution and record the initial reading.
Add $NaOH$ from the burette to the conical flask, swirling continuously.
As the end point approaches, add $NaOH$ dropwise until the indicator changes colour permanently (e.g., phenolphthalein turns from colourless to pale pink).
Record the final burette reading and calculate the volume of $NaOH$ used.
Repeat the titration to obtain concordant results (within $0.10 \text{ cm}^3$ ).

Calculation: Use $n = \frac{cV}{1000}$ and the stoichiometric ratio to find the concentration of $HCl$ .

Mark allocation:

1 mark for correct apparatus (burette, pipette, conical flask, indicator)
1 mark for correct procedure (pipetting acid, titrating with base)
1 mark for identifying the end point via indicator colour change
1 mark for mentioning repetition / concordant results

14. (a) [1 mark]

$CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-$

(b) [2 marks]

The carbonate ion ( $CO_3^{2-}$ ) is the conjugate base of the weak acid $HCO_3^-$ (which is itself the conjugate base of the weak acid $H_2CO_3$ ). Since $CO_3^{2-}$ is a relatively strong conjugate base (from a weak acid), it reacts with water (hydrolysis) to produce $OH^-$ ions. The $Na^+$ ion does not hydrolyse because it comes from the strong base $NaOH$ . The net production of $OH^-$ ions makes the solution basic.

Mark allocation:

1 mark for identifying that $CO_3^{2-}$ is the conjugate base of a weak acid
1 mark for explaining that hydrolysis produces $OH^-$ ions, making the solution basic

15. (a) [2 marks]

$HCl$ is a strong acid and dissociates completely, contributing $0.10 \text{ mol dm}^{-3}$ of $H^+$ ions. Ethanoic acid is a weak acid with $K_a = 1.74 \times 10^{-5}$ , so it contributes only a very small amount of $H^+$ (approximately $\sqrt{1.74 \times 10^{-5} \times 0.10} \approx 1.3 \times 10^{-3} \text{ mol dm}^{-3}$ ). The $H^+$ from $HCl$ is approximately 77 times greater than that from ethanoic acid. Additionally, the high $[H^+]$ from $HCl$ suppresses the dissociation of ethanoic acid (common ion effect), making its contribution even smaller.

Mark allocation:

1 mark for identifying $HCl$ as strong (complete dissociation) and $CH_3COOH$ as weak
1 mark for explaining the common ion effect / that $H^+$ from $HCl$ suppresses $CH_3COOH$ dissociation

(b) [2 marks]

Since the contribution from ethanoic acid is negligible:

$[H^+] \approx 0.10 \text{ mol dm}^{-3} \text{ (from HCl)}$

$\text{pH} = -\log_{10}(0.10) = 1.00$

Mark allocation:

1 mark for stating $[H^+] = 0.10 \text{ mol dm}^{-3}$
1 mark for pH = 1.00

Section C: Application & Data Interpretation

16. (a) [2 marks]

Buffer B will show the smaller pH change.

Explanation: Buffer B has a higher total concentration of buffer components ( $0.20 \text{ mol dm}^{-3}$ each of acid and salt) compared to Buffer A ( $0.10 \text{ mol dm}^{-3}$ each). A buffer with higher concentrations of both components has a greater buffer capacity — it can neutralise more added acid or base before the pH changes significantly.

Mark allocation:

1 mark for identifying Buffer B
1 mark for explaining buffer capacity in terms of higher concentrations

(b) [5 marks]

Before addition of HCl:

$[CH_3COOH] = [CH_3COONa] = 0.10 \text{ mol dm}^{-3}$

$\text{pH} = pK_a + \log_{10}\left(\frac{0.10}{0.10}\right) = 4.76 + \log_{10}(1) = 4.76$

After addition of HCl:

Moles of HCl added $= 0.10 \times \frac{5.0}{1000} = 5.0 \times 10^{-4} \text{ mol}$

Initial moles in $100 \text{ cm}^3$ :

$n(CH_3COOH) = 0.10 \times \frac{100}{1000} = 0.010 \text{ mol}$
$n(CH_3COO^-) = 0.10 \times \frac{100}{1000} = 0.010 \text{ mol}$

The added $H^+$ reacts with $CH_3COO^-$ :

New $n(CH_3COO^-) = 0.010 - 5.0 \times 10^{-4} = 0.0095 \text{ mol}$
New $n(CH_3COOH) = 0.010 + 5.0 \times 10^{-4} = 0.0105 \text{ mol}$

Since total volume is the same for both species (approximately $105 \text{cm}^3$ , but the ratio is unchanged):

$\text{pH} = 4.76 + \log_{10}\left(\frac{0.0095}{0.0105}\right) = 4.76 + \log_{10}(0.9048) = 4.76 - 0.043 = 4.72$

Change in pH $= 4.76 - 4.72 = 0.04$ (very small change, demonstrating buffer action)

Mark allocation:

1 mark for correct initial pH
1 mark for calculating moles of HCl added
1 mark for correct new moles of acid and salt
1 mark for correct final pH
1 mark for correct calculation showing small pH change

17. (a) [1 mark]

Decreasing acid strength: Methanoic acid > Ethanoic acid > Carbonic acid

(or $HCOOH > CH_3COOH > H_2CO_3$ )

(b) [3 marks]

$[H^+]^2 = K_a \times [HCOOH] = 1.6 \times 10^{-4} \times 0.050 = 8.0 \times 10^{-6}$

$[H^+] = \sqrt{8.0 \times 10^{-6}} = 2.828 \times 10^{-3} \text{ mol dm}^{-3}$

$\text{pH} = -\log_{10}(2.828 \times 10^{-3}) = 2.55$

Mark allocation:

1 mark for correct substitution into $K_a$ expression
1 mark for correct $[H^+]$
1 mark for correct pH to 2 d.p.

(c) [1 mark]

The larger the $K_a$ value, the greater the degree of dissociation of the acid in water, and hence the stronger the acid. $K_a$ is a quantitative measure of acid strength: a higher $K_a$ means the equilibrium lies further to the right (more dissociation), producing more $H^+$ ions.

18. (a) [2 marks]

$\text{pOH} = 14.00 - 13.0 = 1.0$

$[OH^-] = 10^{-\text{pOH}} = 10^{-1.0} = 0.10 \text{ mol dm}^{-3}$

Mark allocation:

1 mark for correct pOH
1 mark for correct $[OH^-]$

(b) [4 marks]

$n(NaOH) = 0.10 \times \frac{35.0}{1000} = 3.50 \times 10^{-3} \text{ mol}$

The balanced equation is:

$H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$

Mole ratio: $n(H_2SO_4) : n(NaOH) = 1 : 2$

$n(H_2SO_4) = \frac{3.50 \times 10^{-3}}{2} = 1.75 \times 10^{-3} \text{ mol}$

$[H_2SO_4] = \frac{1.75 \times 10^{-3}}{25.0/1000} = \frac{1.75 \times 10^{-3}}{0.0250} = 0.070 \text{ mol dm}^{-3}$

Mark allocation:

1 mark for moles of NaOH
1 mark for correct stoichiometric ratio (1:2)
1 mark for moles of $H_2SO_4$
1 mark for correct concentration

19. (a) [1 mark]

$n(CH_3COONa) = \frac{8.20}{82.0} = 0.100 \text{ mol}$

(b) [3 marks]

$[CH_3COONa] = \frac{0.100}{0.250} = 0.400 \text{ mol dm}^{-3}$

$[CH_3COOH] = 0.100 \text{ mol dm}^{-3}$

Using the Henderson–Hasselbalch equation:

$\text{pH} = pK_a + \log_{10}\left(\frac{[\text{salt}]}{[\text{acid}]}\right) = 4.76 + \log_{10}\left(\frac{0.400}{0.100}\right)$

$\text{pH} = 4.76 + \log_{10}(4.00) = 4.76 + 0.60 = 5.36$

Mark allocation:

1 mark for correct concentration of salt
1 mark for correct substitution into Henderson–Hasselbalch equation
1 mark for correct final pH

20. (a) [2 marks]

Methyl orange changes colour over the pH range approximately 3.1–4.4 (red to yellow). For a weak acid–strong base titration, the equivalence point occurs at a pH greater than 7 (typically pH 8–9) because the salt formed is basic. Methyl orange would change colour well before the equivalence point is reached, leading to a significant titration error (underestimation of the volume of base needed).

Mark allocation:

1 mark for stating that the equivalence point pH > 7 for weak acid–strong base
1 mark for explaining that methyl orange changes colour in acidic range, causing premature end point

(b) [2 marks]

Phenolphthalein is a more suitable indicator.

Explanation: Phenolphthalein changes colour over the pH range approximately 8.2–10.0 (colourless to pink). This range falls within the steep portion of the titration curve for a weak acid–strong base titration, which occurs around pH 7–10. The colour change therefore coincides with the equivalence point, giving an accurate result.

Mark allocation:

1 mark for suggesting phenolphthalein
1 mark for explaining that its pH range matches the equivalence point region

End of Answer Key

Total Marks: 50