A Level H1 Chemistry Practice Paper 2

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A-Level Chemistry H1 Quiz - Acids Bases Salts

Name: ____________________
Class: ____________________
Date: ____________________**
Score: ________ / 45

Duration: 60 Minutes
Total Marks: 45
Instructions: Answer all questions. Show all working for calculations. Use the data booklet provided.

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Section A: Foundational Concepts (Questions 1–7)

1. What is meant by the term weak acid? [Illustrate your answer with an equation.]
   
   
   (2 marks)

2. State the definition of a Brønsted-Lowry base.
   
   (1 mark)

3. Identify the Period 3 element that forms a sparingly soluble amphoteric oxide.
   
   (1 mark)

4. Write the balanced equation, including state symbols, for the dissociation of ethanoic acid in aqueous solution.
   
   
   (2 marks)

5. Explain why a solution of sodium chloride has a pH of approximately 7.0, whereas a solution of sodium ethanoate has a pH greater than 7.0.
   
   
   (2 marks)

6. Define the term buffer solution.
   
   
   (2 marks)

7. Which of the following is a strong base? A) $\text{NH}_3$ B) $\text{Mg(OH)}_2$ C) $\text{KOH}$ D) $\text{Al(OH)}_3$ (1 mark)

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Section B: Calculations and Equilibrium (Questions 8–15)

8. (a) Construct a balanced equation, including state symbols, for the first dissociation of carbonic acid ($\text{H}_2\text{CO}_3$) in rainwater. [2]
   
   (b) Write the expression for the acid dissociation constant ($K_a$) for this reaction. [1]
   
   (3 marks)

9. Calculate the pH of a $0.10\text{ mol dm}^{-3}$ solution of nitric acid ($\text{HNO}_3$).
   
   
   (2 marks)

10. A $0.050\text{ mol dm}^{-3}$ solution of a weak monoprotic acid HA has a $K_a$ of $1.8 \times 10^{-5}$. Calculate the $[\text{H}^+]$ concentration of this solution.
    
    
    (3 marks)

11. Using the answer in Question 10, calculate the pH of the solution.
    
    
    (2 marks)

12. A $25.0\text{ cm}^3$ sample of benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$) was titrated against a standardized $0.100\text{ mol dm}^{-3}$ solution of $\text{NaOH}$. The average titre volume was $18.50\text{ cm}^3$. Calculate the concentration of the benzoic acid solution.
    
    
    
    (3 marks)

13. Calculate the mass of $\text{NaOH}$ required to prepare $250\text{ cm}^3$ of a $0.20\text{ mol dm}^{-3}$ solution.
    
    
    (2 marks)

14. For the dissociation of a weak acid $\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$, explain how the pH changes if the concentration of the acid is doubled.
    
    
    (2 marks)

15. A buffer solution is prepared by mixing $0.10\text{ mol}$ of $\text{CH}_3\text{COOH}$ and $0.10\text{ mol}$ of $\text{CH}_3\text{COONa}$ in $1.0\text{ dm}^3$ of water. If the $pK_a$ of ethanoic acid is 4.76, calculate the pH of this buffer.
    
    
    (3 marks)

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

16. Calcium hydroxide is added to fermentation tanks to prevent the buildup of lactic acid. Why does high acidity (low pH) reduce the effectiveness of the enzymes involved in fermentation?
    
    
    (2 marks)

17. A student is given a solution of an unknown oxide of a Period 3 element. The oxide reacts with both $2.0\text{ mol dm}^{-3}\text{ HCl}$ and $2.0\text{ mol dm}^{-3}\text{ NaOH}$. (a) What is the nature of this oxide? [1] (b) Identify the element. [1] (2 marks)

18. Compare the pH of $0.1\text{ mol dm}^{-3}\text{ HCl}$ and $0.1\text{ mol dm}^{-3}\text{ CH}_3\text{COOH}$. Explain your answer in terms of dissociation.
    
    
    (3 marks)

19. A sample of a diprotic acid $\text{H}_2\text{A}$ has $K_{a1} = 1.0 \times 10^{-3}$ and $K_{a2} = 1.0 \times 10^{-7}$. Which dissociation step is more favorable? Explain why.
    
    
    (2 marks)

20. Describe the change in pH that occurs during the titration of a weak acid with a strong base, specifically at the equivalence point.
    
    
    (3 marks)

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Answer Key - A-Level Chemistry H1 Quiz: Acids Bases Salts

Q	Answer	Marks	Marking Notes
1	A weak acid is an acid that only partially dissociates/ionizes in aqueous solution. $\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^- \text{(aq)} + \text{H}^+ \text{(aq)}$	2	1m for definition, 1m for equation with $\rightleftharpoons$ and state symbols.
2	A proton ($\text{H}^+$) acceptor.	1	Must mention proton acceptor.
3	Aluminium (Al)	1	$\text{Al}_2\text{O}_3$ is the oxide.
4	$\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{CH}_3\text{COO}^- \text{(aq)} + \text{H}^+ \text{(aq)}$	2	1m for $\rightleftharpoons$, 1m for correct species and (aq).
5	$\text{NaCl}$ is a salt of a strong acid ($\text{HCl}$) and strong base ($\text{NaOH}$), so ions do not hydrolyze. $\text{CH}_3\text{COONa}$ contains $\text{CH}_3\text{COO}^-$, which is a conjugate base of a weak acid and reacts with water (hydrolysis) to produce $\text{OH}^-$.	2	1m for $\text{NaCl}$ neutral, 1m for $\text{CH}_3\text{COO}^-$ hydrolysis.
6	A solution that resists significant changes in pH when small amounts of acid or base are added.	2	1m for "resists change", 1m for "small amounts of acid/base".
7	C) $\text{KOH}$	1	Group 1 hydroxides are strong bases.
8	(a) $\text{H}_2\text{CO}_3\text{(aq)} \rightleftharpoons \text{HCO}_3^-\text{(aq)} + \text{H}^+\text{(aq)}$ <br> (b) $K_a = \frac{[\text{HCO}_3^-][\text{H}^+]}{[\text{H}_2\text{CO}_3]}$	3	(a) 1m for $\rightleftharpoons$, 1m for state symbols. (b) 1m for correct expression.
9	$\text{pH} = -\log(0.10) = 1.0$	2	1m for formula/substitution, 1m for answer.
10	$[\text{H}^+] = \sqrt{K_a \times c} = \sqrt{1.8 \times 10^{-5} \times 0.050} = \sqrt{9.0 \times 10^{-7}} = 9.49 \times 10^{-4}\text{ mol dm}^{-3}$	3	1m for formula, 1m for substitution, 1m for correct value.
11	$\text{pH} = -\log(9.49 \times 10^{-4}) = 3.02$	2	1m for substitution, 1m for answer.
12	$n(\text{NaOH}) = 0.100 \times (18.50/1000) = 0.00185\text{ mol}$ <br> $n(\text{Acid}) = 0.00185\text{ mol}$ (1:1 ratio) <br> $c(\text{Acid}) = 0.00185 / (25.0/1000) = 0.074\text{ mol dm}^{-3}$	3	1m for moles of base, 1m for mole ratio, 1m for final concentration.
13	$n = 0.20 \times 0.250 = 0.050\text{ mol}$ <br> $m = 0.050 \times 40.0 = 2.0\text{ g}$	2	1m for moles, 1m for mass.
14	The pH will decrease. As concentration increases, the equilibrium $\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$ shifts to the right, increasing $[\text{H}^+]$.	2	1m for pH decrease, 1m for equilibrium shift/increased $[\text{H}^+]$.
15	$\text{pH} = pK_a + \log(\frac{[\text{salt}]}{[\text{acid}]}) = 4.76 + \log(\frac{0.10}{0.10}) = 4.76 + 0 = 4.76$	3	1m for Henderson-Hasselbalch, 1m for substitution, 1m for answer.
16	High acidity denatures the enzyme by disrupting ionic/hydrogen bonds in the tertiary structure. This changes the shape of the active site, so the substrate cannot bind.	2	1m for denaturation/structure change, 1m for active site/substrate binding.
17	(a) Amphoteric <br> (b) Aluminium	2	1m for amphoteric, 1m for Al.
18	$\text{HCl}$ has a lower pH than $\text{CH}_3\text{COOH}$. $\text{HCl}$ is a strong acid and dissociates completely, providing a higher $[\text{H}^+]$, while $\text{CH}_3\text{COOH}$ is a weak acid and dissociates only partially.	3	1m for pH comparison, 1m for strong/complete dissociation, 1m for weak/partial dissociation.
19	The first dissociation step is more favorable. $K_{a1} \gg K_{a2}$, meaning the first proton is much more easily lost than the second.	2	1m for first step, 1m for comparison of $K_a$ values.
20	The pH at the equivalence point is $> 7$. This is because the conjugate base of the weak acid ($\text{A}^-$) reacts with water (hydrolysis) to produce $\text{OH}^-$ ions.	3	1m for $\text{pH} > 7$, 1m for conjugate base hydrolysis, 1m for production of $\text{OH}^-$.