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A Level H1 Chemistry Practice Paper 1

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TuitionGoWhere Practice Paper - Chemistry H1 A-Level

TuitionGoWhere Practice Paper (AI)

Subject: Chemistry H1
Level: A-Level
Paper: Practice Paper 1 (Version 1)
Duration: 2 Hours
Total Marks: 80
Name: __________________________ Class: __________ Date: __________

Instructions to Candidates

Answer all questions.
Write your answers in the spaces provided.
Use a black or dark blue pen.
For calculations, show all working. Give your numerical answers to 3 significant figures unless otherwise stated.
The following data is provided:
- $K_w = 1.00 \times 10^{-14} \text{ mol}^2 \text{ dm}^{-6}$ at 298 K
- $R = 8.31 \text{ J mol}^{-1} \text{ K}^{-1}$

Section A: Structured Questions (40 Marks)

Question 1 (a) Define the term Brønsted-Lowry base. [1]

(b) Consider the reaction: $\text{NH}_3(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{NH}_4^+(\text{aq}) + \text{OH}^-(\text{aq})$ (i) Identify the conjugate acid-base pairs in this reaction. [2] Pair 1: ____________________________________________________________________ Pair 2: ____________________________________________________________________

(ii) Explain why $\text{NH}_3$ is described as a weak base. [2]

(c) Calculate the pH of a $0.10 \text{ mol dm}^{-3}$ solution of ammonia, given that $K_b = 1.8 \times 10^{-5} \text{ mol dm}^{-3}$ . [3]

Question 2 (a) State the meaning of the term amphoteric oxide. [1]

(b) Aluminium oxide, $\text{Al}_2\text{O}_3$ , is an amphoteric oxide. (i) Write a balanced chemical equation, including state symbols, for the reaction of $\text{Al}_2\text{O}_3$ with hot, concentrated sodium hydroxide solution. [2]

(ii) Write a balanced chemical equation, including state symbols, for the reaction of $\text{Al}_2\text{O}_3$ with hydrochloric acid. [2]

Question 3 (a) A student titrates $25.0 \text{ cm}^3$ of a solution of benzoic acid ( $\text{C}_6\text{H}_5\text{COOH}$ ) with $0.100 \text{ mol dm}^{-3}$ sodium hydroxide ( $\text{NaOH}$ ). The average titre volume of $\text{NaOH}$ required to reach the end-point is $18.50 \text{ cm}^3$ . (i) Calculate the amount, in moles, of $\text{NaOH}$ used in the titration. [1]

(ii) Determine the concentration of the benzoic acid solution in $\text{mol dm}^{-3}$ . [2]

(b) Benzoic acid is a weak acid with $pK_a = 4.20$ . Calculate the pH of a $0.050 \text{ mol dm}^{-3}$ solution of benzoic acid. [3]

Question 4 (a) What is a buffer solution? [1]

(b) A buffer solution is prepared by mixing $50.0 \text{ cm}^3$ of $0.20 \text{ mol dm}^{-3}$ ethanoic acid ( $\text{CH}_3\text{COOH}$ ) and $50.0 \text{ cm}^3$ of $0.20 \text{ mol dm}^{-3}$ sodium ethanoate ( $\text{CH}_3\text{COONa}$ ). (i) Calculate the pH of this buffer solution. ( $pK_a$ of ethanoic acid = 4.76) [2]

(ii) Explain, with the aid of an equation, how the pH of this buffer remains relatively constant when a small amount of $\text{HCl}(\text{aq})$ is added. [3]

Question 5 (a) Define the solubility product, $K_{sp}$ . [2]

(b) The $K_{sp}$ of $\text{CaF}_2$ is $3.9 \times 10^{-11} \text{ mol}^3 \text{ dm}^{-9}$ at 298 K. (i) Write the expression for $K_{sp}$ of $\text{CaF}_2$ . [1]

(ii) Calculate the solubility of $\text{CaF}_2$ in pure water in $\text{mol dm}^{-3}$ . [3]

(c) Predict and explain the effect on the solubility of $\text{CaF}_2$ if $0.10 \text{ mol dm}^{-3} \text{NaF}(\text{aq})$ is added to the solution. [2]

Section B: Extended Response (40 Marks)

Question 6 (a) Carbonic acid, $\text{H}_2\text{CO}_3$ , is a diprotic weak acid found in rainwater. (i) Write the balanced equation for the first dissociation of $\text{H}_2\text{CO}_3$ in water. [1]

(ii) Given $K_{a1} = 4.3 \times 10^{-7} \text{ mol dm}^{-1}$ , calculate the pH of a $0.010 \text{ mol dm}^{-3}$ solution of $\text{H}_2\text{CO}_3$ . [3]

(iii) Explain why the second dissociation constant, $K_{a2}$ , is significantly smaller than $K_{a1}$ . [2]

(b) Discuss the role of the bicarbonate buffer system ( $\text{H}_2\text{CO}_3 / \text{HCO}_3^-$ ) in maintaining the pH of human blood. Include relevant chemical equations. [5]

Question 7 (a) Compare the strengths of $\text{HCl}$ , $\text{CH}_3\text{COOH}$ , and $\text{HCN}$ . Arrange them in order of increasing acid strength and justify your answer based on the stability of the conjugate base or bond polarity. [4]

(b) A solution is formed by mixing $100 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3} \text{NaOH}$ and $100 \text{ cm}^3$ of $0.10 \text{ mol dm}^{-3} \text{CH}_3\text{COOH}$ . (i) Calculate the pH of the resulting solution. ( $pK_a$ of ethanoic acid = 4.76) [4]

(ii) Identify the type of solution formed in (b)(i) and explain why. [2]

(c) Describe a method to determine the $K_{sp}$ of a sparingly soluble salt, such as $\text{AgCl}$ , using a titration method. [5]

Question 8 (a) Explain the difference between a strong base and a concentrated base. [2]

(b) For the reaction: $\text{H}_2\text{SO}_4(\text{aq}) + 2\text{KOH}(\text{aq}) \to \text{K}_2\text{SO}_4(\text{aq}) + 2\text{H}_2\text{O}(\text{l})$ (i) If $25.0 \text{ cm}^3$ of $\text{H}_2\text{SO}_4$ requires $30.0 \text{ cm}^3$ of $0.20 \text{ mol dm}^{-3} \text{KOH}$ for neutralization, calculate the concentration of the $\text{H}_2\text{SO}_4$ solution. [3]

(ii) Calculate the mass of $\text{K}_2\text{SO}_4$ produced in this reaction. [3]

(c) A sample of an unknown salt is found to be soluble in both $\text{HCl}(\text{aq})$ and $\text{NaOH}(\text{aq})$ . (i) What can you conclude about the nature of this salt? [1]

(ii) Suggest a possible identity for the metal cation in this salt. [1]

(d) Explain why the pH of a $0.10 \text{ mol dm}^{-3}$ solution of $\text{NaCl}$ is approximately 7.0, while the pH of a $0.10 \text{ mol dm}^{-3}$ solution of $\text{CH}_3\text{COONa}$ is greater than 7.0. [5]

Answers

Answer Key - Chemistry H1 Practice Paper 1 (Version 1)

Section A: Structured Questions

Question 1 (a) A substance that can accept a proton ( $\text{H}^+$ ). [1] (b) (i) Pair 1: $\text{NH}_3$ (base) / $\text{NH}_4^+$ (acid); Pair 2: $\text{H}_2\text{O}$ (acid) / $\text{OH}^-$ (base). [2] (ii) It only partially dissociates/ionizes in aqueous solution. [1] It exists in equilibrium with its molecular form. [1] (c) $[\text{OH}^-] = \sqrt{K_b \times c} = \sqrt{1.8 \times 10^{-5} \times 0.10} = 1.34 \times 10^{-3} \text{ mol dm}^{-3}$ . [1] $\text{pOH} = -\log(1.34 \times 10^{-3}) = 2.87$ . [1] $\text{pH} = 14 - 2.87 = 11.13$ . [1]

Question 2 (a) An oxide that reacts with both acids and bases to form salt and water. [1] (b) (i) $\text{Al}_2\text{O}_3(\text{s}) + 2\text{NaOH}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \to 2\text{Na}[\text{Al}(\text{OH})_4](\text{aq})$ (or $\text{NaAlO}_2$ version). [2] (ii) $\text{Al}_2\text{O}_3(\text{s}) + 6\text{HCl}(\text{aq}) \to 2\text{AlCl}_3(\text

<stage5_exam_answers_md>
# Answer Key - Chemistry H1 Practice Paper 1 (Version 1)

### Section A: Structured Questions

**Question 1**
(a) A substance that can accept a proton ($\text{H}^+$). [1]
(b) (i) Pair 1: $\text{NH}_3$ (base) / $\text{NH}_4^+$ (acid); Pair 2: $\text{H}_2\text{O}$ (acid) / $\text{OH}^-$ (base). [2]
(ii) It only partially dissociates/ionizes in aqueous solution. [1] It exists in equilibrium with its molecular form. [1]
(c) $[\text{OH}^-] = \sqrt{K_b \times c} = \sqrt{1.8 \times 10^{-5} \times 0.10} = 1.34 \times 10^{-3} \text{ mol dm}^{-3}$. [1]
$\text{pOH} = -\log(1.34 \times 10^{-3}) = 2.87$. [1]
$\text{pH} = 14 - 2.87 = 11.13$. [1]

**Question 2**
(a) An oxide that reacts with both acids and bases to form salt and water. [1]
(b) (i) $\text{Al}_2\text{O}_3(\text{s}) + 2\text{NaOH}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \to 2\text{Na}[\text{Al}(\text{OH})_4](\text{aq})$ [2]
(ii) $\text{Al}_2\text{O}_3(\text{s}) + 6\text{HCl}(\text{aq}) \to 2\text{AlCl}_3(\text{aq}) + 3\text{H}_2\text{O}(\text{l})$ [2]
(c) It has a high lattice energy/strong ionic bonding that cannot be overcome by the hydration energy of water. [1]

**Question 3**
(a) (i) $n(\text{NaOH}) = 0.100 \times (18.50/1000) = 1.85 \times 10^{-3} \text{ mol}$. [1]
(ii) $n(\text{benzoic acid}) = 1.85 \times 10^{-3} \text{ mol}$.
$\text{Concentration} = (1.85 \times 10^{-3}) / (25.0/1000) = 0.0740 \text{ mol dm}^{-3}$. [2]
(b) $[\text{H}^+] = \sqrt{K_a \times c} = \sqrt{10^{-4.20} \times 0.050} = \sqrt{6.31 \times 10^{-5} \times 0.050} = 1.78 \times 10^{-3} \text{ mol dm}^{-3}$. [1]
$\text{pH} = -\log(1.78 \times 10^{-3}) = 2.75$. [2]

**Question 4**
(a) A solution that resists significant changes in pH when small amounts of acid or base are added. [1]
(b) (i) Since $[\text{acid}] = [\text{salt}]$, $\text{pH} = pK_a = 4.76$. [2]
(ii) $\text{CH}_3\text{COO}^- + \text{H}^+ \to \text{CH}_3\text{COOH}$. [2] The ethanoate ions react with added $\text{H}^+$ to prevent a large increase in $[\text{H}^+]$. [1]

**Question 5**
(a) The equilibrium constant for a saturated solution of a sparingly soluble salt. [2]
(b) (i) $K_{sp} = [\text{Ca}^{2+}][\text{F}^-]^2$. [1]
(ii) $K_{sp} = s(2s)^2 = 4s^3 \implies s = \sqrt[3]{(3.9 \times 10^{-11})/4} = 2.14 \times 10^{-4} \text{ mol dm}^{-3}$. [3]
(c) Solubility decreases. [1] Common ion effect: adding $\text{F}^-$ shifts the equilibrium to the left (precipitate forms). [1]

### Section B: Extended Response

**Question 6**
(a) (i) $\text{H}_2\text{CO}_3(\text{aq}) \rightleftharpoons \text{H}^+(\text{aq}) + \text{HCO}_3^-(\text{aq})$. [1]
(ii) $[\text{H}^+] = \sqrt{4.3 \times 10^{-7} \times 0.010} = 6.56 \times 10^{-5} \text{ mol dm}^{-3}$. $\text{pH} = 4.18$. [3]
(iii) The second dissociation involves removing a proton from a negatively charged ion ($\text{HCO}_3^-$), which is more strongly attracted to the proton than the neutral $\text{H}_2\text{CO}_3$. [2]
(b) $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$. [2] If blood becomes too acidic, $\text{HCO}_3^-$ reacts with $\text{H}^+$. If too basic, $\text{H}_2\text{CO}_3$ dissociates. [3]

**Question 7**
(a) $\text{HCN} < \text{CH}_3\text{COOH} < \text{HCl}$. [1] $\text{HCl}$ is strong (complete dissociation). [1] $\text{CH}_3\text{COOH}$ is weak (stabilized by resonance of conjugate base). [1] $\text{HCN}$ is very weak (strong $\text{H-C}$ bond/low polarity). [1]
(b) (i) Equimolar amounts of weak acid and strong base form a salt of a weak acid.
$[\text{CH}_3\text{COO}^-] = 0.10 / 2 = 0.05 \text{ mol dm}^{-3}$.
$K_b = K_w / K_a = 10^{-14} / 10^{-4.76} = 1.74 \times 10^{-10}$.
$[\text{OH}^-] = \sqrt{1.74 \times 10^{-10} \times 0.05} = 2.95 \times 10^{-6}$. $\text{pOH} = 5.53 \implies \text{pH} = 8.47$. [4]
(ii) Basic salt solution. [1] The ethanoate ion undergoes hydrolysis. [1]
(c) Prepare a saturated solution of $\text{AgCl}$. [1] Titrate a known volume against a standard solution of $\text{AgNO}_3$ or use a precipitation titration (Volhard method). [2] Calculate $[\text{Cl}^-]$ and $[\text{Ag}^+]$. [1] $K_{sp} = [\text{Ag}^+][\text{Cl}^-]$. [1]

**Question 8**
(a) Strong base: completely dissociates in water. [1] Concentrated base: high molarity/amount of solute per volume. [1]
(b) (i) $n(\text{KOH}) = 0.20 \times 0.030 = 6.0 \times 10^{-3} \text{ mol}$.
$n(\text{H}_2\text{SO}_4) = 6.0 \times 10^{-3} / 2 = 3.0 \times 10^{-3} \text{ mol}$.
$\text{Conc} = 3.0 \times 10^{-3} / 0.025 = 0.12 \text{ mol dm}^{-3}$. [3]
(ii) $n(\text{K}_2\text{SO}_4) = 3.0 \times 10^{-3} \text{ mol}$.
$\text{Mass} = 3.0 \times 10^{-3} \times 174.3 = 0.523 \text{ g}$. [3]
(c) (i) Amphoteric. [1] (ii) $\text{Al}^{3+}$ or $\text{Zn}^{2+}$. [1]
(d) $\text{NaCl}$ is formed from a strong acid and strong base; neither ion hydrolyzes. [2] $\text{CH}_3\text{COONa}$ contains $\text{CH}_3\text{COO}^-$, which is a conjugate base of a weak acid. [2] It reacts with water: $\text{CH}_3\text{COO}^- + \text{H}_2\text{O} \rightleftharpoons \text{CH}_3\text{COOH} + \text{OH}^-$, increasing pH. [1]