A Level H1 Chemistry Acids Bases Salts Quiz

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A-Level Chemistry H1 Quiz - Acids Bases Salts

Name: ________________________
Class: ________________________
Date: ________________________
Score: ________ / 50

Duration: 60 Minutes
Total Marks: 50
Instructions: Answer all questions. Show all working for calculations. Use the provided data booklet for $K_w$ and $K_a$ values where necessary.

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Section A: Foundational Concepts (Questions 1–5)

1. Define the term weak acid and provide a balanced chemical equation, including state symbols, for the dissociation of ethanoic acid in water. [2]
   
   
   
   
   
   
   
   
   
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2. Distinguish between a strong acid and a concentrated acid. [2]
   
   
   
   
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3. Identify the Period 3 element that forms a sparingly soluble amphoteric oxide. [1]
   
   
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4. State the Brønsted-Lowry definition of a base. [1]
   
   
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5. Explain why the pH of a $0.10\text{ mol dm}^{-3}$ solution of $\text{HCl}$ is lower than the pH of a $0.10\text{ mol dm}^{-3}$ solution of $\text{CH}_3\text{COOH}$. [2]
   
   
   
   
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Section B: Quantitative Analysis & Calculations (Questions 6–15)

6. Calculate the pH of a $0.050\text{ mol dm}^{-3}$ solution of nitric acid ($\text{HNO}_3$). [1]
   
   
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7. A $25.0\text{ cm}^3$ sample of a weak monoprotic acid $\text{HA}$ was titrated against $0.100\text{ mol dm}^{-3}$ $\text{NaOH}$. The average titre volume was $18.20\text{ cm}^3$. Calculate the concentration of the acid. [3]
   
   
   
   
   
   
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8. For the acid in Question 7, the pH of the solution was found to be 3.20. Calculate the acid dissociation constant, $K_a$, for $\text{HA}$. [3]
   
   
   
   
   
   
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9. Write the expression for the acid dissociation constant, $K_a$, for the first dissociation of carbonic acid ($\text{H}_2\text{CO}_3$). [1]
   
   
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10. Calculate the pH of a $0.010\text{ mol dm}^{-3}$ solution of ammonia ($\text{NH}_3$), given that $K_b = 1.8 \times 10^{-5}\text{ mol dm}^{-3}$. [3]
    
    
    
    
    
    
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11. A buffer solution is prepared by mixing $0.15\text{ mol dm}^{-3}$ propanoic acid ($\text{CH}_3\text{CH}_2\text{COOH}$) and $0.25\text{ mol dm}^{-3}$ sodium propanoate. Given $pK_a = 4.87$, calculate the pH of this buffer. [3]
    
    
    
    
    
    
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12. How would the pH of the buffer in Question 11 change if a small amount of $0.1\text{ mol dm}^{-3}$ $\text{HCl}$ is added? Explain your answer using a chemical equation. [3]
    
    
    
    
    
    
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13. Calculate the mass of $\text{Na}_2\text{CO}_3$ required to prepare $250\text{ cm}^3$ of a $0.020\text{ mol dm}^{-3}$ solution. [2]
    
    
    
    
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14. A solution of a weak acid $\text{HA}$ has a $K_a$ of $1.0 \times 10^{-5}$. If the concentration of the acid is doubled, what happens to the pH? Justify your answer. [2]
    
    
    
    
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15. Determine the pH of a solution formed by mixing $50\text{ cm}^3$ of $0.10\text{ mol dm}^{-3}$ $\text{HCl}$ and $50\text{ cm}^3$ of $0.10\text{ mol dm}^{-3}$ $\text{NaOH}$. [2]
    
    
    
    
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Section C: Application & Reasoning (Questions 16–20)

16. In a fermentation tank, calcium hydroxide is added to prevent the buildup of lactic acid. Explain why excessive acidity (low pH) would reduce the effectiveness of the enzymes involved in fermentation. [2]
    
    
    
    
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17. Compare the solubility of $\text{Al}_2\text{O}_3$ in $\text{HCl}(\text{aq})$ and $\text{NaOH}(\text{aq})$. Provide one balanced equation to support your answer. [2]
    
    
    
    
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18. A salt is formed by the reaction between a strong acid and a weak base. Predict whether the resulting solution will be acidic, basic, or neutral. Explain your reasoning. [2]
    
    
    
    
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19. Describe the effect of increasing the temperature on the value of $K_w$ for water. How does this affect the neutrality of water at $60^\circ\text{C}$? [2]
    
    
    
    
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20. An unknown salt $X$ is found to be insoluble in water but dissolves in both concentrated $\text{HCl}$ and concentrated $\text{NaOH}$. Identify the nature of the oxide/salt $X$ and give an example of such a compound. [2]
    
    
    
    
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A-Level Chemistry H1 Quiz - Acids Bases Salts (Answer Key)

Section A: Foundational Concepts

1. Definition: A weak acid is an acid that only partially dissociates/ionizes in aqueous solution. [1] Equation: $\text{CH}_3\text{COOH}(\text{aq}) \rightleftharpoons \text{CH}_3\text{COO}^-(\text{aq}) + \text{H}^+(\text{aq})$ (Must have equilibrium arrow and state symbols). [1]
2. Strong acid: Completely dissociates in water to produce $\text{H}^+$ ions. [1] Concentrated acid: Has a high molarity/concentration of solute per unit volume of solvent. [1]
3. Aluminium (Al). [1]
4. Brønsted-Lowry Base: A species (molecule or ion) that acts as a proton ($\text{H}^+$) acceptor. [1]
5. $\text{HCl}$ is a strong acid and dissociates completely, resulting in a higher concentration of $\text{H}^+$ ions compared to $\text{CH}_3\text{COOH}$, which is a weak acid and only partially dissociates. Since $\text{pH} = -\log[\text{H}^+]$, a higher $[\text{H}^+]$ leads to a lower pH. [2]

Section B: Quantitative Analysis & Calculations

6. $[\text{H}^+] = 0.050\text{ mol dm}^{-3}$; $\text{pH} = -\log(0.050) = 1.30$. [1]
7. $\text{n}(\text{NaOH}) = 0.100 \times (18.20/1000) = 0.00182\text{ mol}$. [1] $\text{n}(\text{HA}) = 0.00182\text{ mol}$ (1:1 ratio). [1] $\text{Conc}(\text{HA}) = 0.00182 / (25.0/1000) = 0.0728\text{ mol dm}^{-3}$. [1]
8. $\text{pH} = 3.20 \implies [\text{H}^+] = 10^{-3.20} = 6.31 \times 10^{-4}\text{ mol dm}^{-3}$. [1] $K_a = [\text{H}^+]^2 / [\text{HA}] = (6.31 \times 10^{-4})^2 / 0.0728$. [1] $K_a = 5.47 \times 10^{-6}\text{ mol dm}^{-3}$. [1]
9. $K_a = \frac{[\text{HCO}_3^-][\text{H}^+]}{[\text{H}_2\text{CO}_3]}$ [1]
10. $[\text{OH}^-] = \sqrt{K_b \times \text{conc}} = \sqrt{1.8 \times 10^{-5} \times 0.010} = 4.24 \times 10^{-4}\text{ mol dm}^{-3}$. [1] $\text{pOH} = -\log(4.24 \times 10^{-4}) = 3.37$. [1] $\text{pH} = 14 - 3.37 = 10.63$. [1]
11. $\text{pH} = pK_a + \log([\text{salt}]/[\text{acid}]) = 4.87 + \log(0.25/0.15)$. [1] $\text{pH} = 4.87 + 0.22 = 5.09$. [2]
12. The pH will decrease slightly. [1] The added $\text{H}^+$ reacts with the conjugate base: $\text{CH}_3\text{CH}_2\text{COO}^- + \text{H}^+ \to \text{CH}_3\text{CH}_2\text{COOH}$. [2]
13. $\text{n} = 0.020 \times 0.250 = 0.005\text{ mol}$. [1] $\text{Mass} = 0.005 \times 106.0 = 0.53\text{ g}$. [1]
14. The pH will decrease. [1] Since $[\text{H}^+] \approx \sqrt{K_a \times [\text{HA}]}$, increasing the concentration of the weak acid increases the concentration of $\text{H}^+$ ions produced. [1]
15. $\text{n}(\text{HCl}) = 0.005\text{ mol}$; $\text{n}(\text{NaOH}) = 0.005\text{ mol}$. [1] They neutralize completely to form $\text{NaCl}$ and $\text{H}_2\text{O}$. The solution is neutral, $\text{pH} = 7.0$. [1]

Section C: Application & Reasoning

16. Low pH (high acidity) denatures the enzyme by disrupting the ionic and hydrogen bonds in its tertiary structure. [1] This changes the shape of the active site, preventing the substrate from binding. [1]
17. $\text{Al}_2\text{O}_3$ is amphoteric; it is soluble in both. [1] Equation: $\text{Al}_2\text{O}_3(\text{s}) + 6\text{HCl}(\text{aq}) \to 2\text{AlCl}_3(\text{aq}) + 3\text{H}_2\text{O}(\text{l})$ OR $\text{Al}_2\text{O}_3(\text{s}) + 2\text{NaOH}(\text{aq}) + 3\text{H}_2\text{O}(\text{l}) \to 2\text{Na}[\text{Al}(\text{OH})_4](\text{aq})$. [1]
18. Acidic. [1] The conjugate acid of the weak base will undergo hydrolysis in water, releasing $\text{H}^+$ ions. [1]
19. $K_w$ increases as temperature increases (dissociation of water is endothermic). [1] The pH of neutral water will be lower than 7.0 (e.g., $\sim 6.5$), but it remains "neutral" as $[\text{H}^+] = [\text{OH}^-]$. [1]
20. Amphoteric. [1] Example: Aluminium oxide ($\text{Al}_2\text{O}_3$) or Zinc oxide ($\text{ZnO}$). [1]