Question
This question is about acid–base properties.
(a) (i) Deduce the ionic equation, including state symbols, for the reaction of hydrogen chloride gas with water.
▶️Answer/Explanation
Ans:
The reaction of hydrogen chloride gas (HCl) with water (H₂O) involves the HCl molecule donating a proton (H⁺) to the water molecule, resulting in the formation of hydronium ion (H₃O⁺) and chloride ion (Cl⁻). The ionic equation for this reaction, including state symbols, is as follows:
\[ \text{HCl(g)} + \text{H₂O(l)} \rightarrow \text{H₃O⁺(aq)} + \text{Cl⁻(aq)} \]
This equation represents the dissociation of hydrogen chloride gas in water to produce hydronium ions and chloride ions in aqueous solution.
Question
(ii) Calculate the pH of 0.50mol \(dm^{-3}\) hydrochloric acid.
▶️Answer/Explanation
Ans:
Hydrochloric acid (\(HCl\)) is a strong acid that dissociates completely in water to produce \(H^+\) ions and \(Cl^-\) ions. Since \(HCl\) is a strong acid, the concentration of \(H^+\) ions in the resulting solution will be equal to the initial concentration of hydrochloric acid (\(0.50 \, \text{mol} \, dm^{-3}\)).
The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration (\(H^+\)) in moles per liter (\(mol \, dm^{-3}\)). Mathematically, it is expressed as:
\[ \text{pH} = -\log[H^+] \]
Given that the concentration of \(H^+\) ions in the hydrochloric acid solution is \(0.50 \, mol \, dm^{-3}\), we can calculate the pH as follows:
\[ \text{pH} = -\log(0.50) \]
Using a calculator:
\[ \text{pH} = -\log(0.50) \]
\[ \text{pH} \approx -(-0.301) \]
\[ \text{pH} \approx 0.301 \]
Therefore, the pH of a \(0.50 \, mol \, dm^{-3}\) hydrochloric acid solution is approximately \(0.301\).
Question
(iii) Explain why a solution of ethanoic acid has a higher pH than hydrochloric acid of the same concentration.
▶️Answer/Explanation
Ans:
Ethanoic acid (also known as acetic acid) is a weak acid, while hydrochloric acid is a strong acid. The difference in pH between solutions of ethanoic acid and hydrochloric acid of the same concentration arises from their different dissociation behaviors.
1. Weak Acid Dissociation:
- Ethanoic acid (\(CH_3COOH\)) is a weak acid, meaning it only partially dissociates in water to produce \(H^+\) ions and \(CH_3COO^-\) ions. The dissociation equilibrium is represented by the equation:
\[ CH_3COOH(aq) + H_2O(l) \rightleftharpoons CH_3COO^-(aq) + H_3O^+(aq) \] - Because ethanoic acid only partially dissociates, the concentration of \(H^+\) ions in the resulting solution is lower compared to a solution of the same concentration of a strong acid like hydrochloric acid.
2. Strong Acid Dissociation:
- Hydrochloric acid (\(HCl\)) is a strong acid, which means it completely dissociates in water to produce \(H^+\) ions and \(Cl^-\) ions. The dissociation of hydrochloric acid is complete, as represented by the equation:
\[ HCl(aq) \rightarrow H^+(aq) + Cl^-(aq) \] - Because hydrochloric acid completely dissociates, the concentration of \(H^+\) ions in the resulting solution is equal to the concentration of the acid, which in this case is \(0.50 \, mol \, dm^{-3}\).
3. pH Calculation:
- The pH of a solution is determined by the concentration of \(H^+\) ions. Since ethanoic acid only partially dissociates, the concentration of \(H^+\) ions in the resulting solution is lower compared to hydrochloric acid of the same concentration. As a result, the pH of a solution of ethanoic acid is higher than the pH of hydrochloric acid of the same concentration.
In summary, a solution of ethanoic acid has a higher pH than hydrochloric acid of the same concentration because ethanoic acid is a weak acid and only partially dissociates, leading to a lower concentration of \(H^+\) ions in the solution.
Question
(iv) A pH probe can be used to distinguish between the acids in part (a)(iii). Identify another simple instrumental method that could be used in a school laboratory to distinguish between the two acids.
▶️Answer/Explanation
Ans:
Another simple instrumental method that could be used in a school laboratory to distinguish between the two acids is conductivity measurement.
Principle:
- Conductivity is a measure of a solution’s ability to conduct an electric current. Strong acids, which dissociate completely into ions in solution, result in a higher conductivity compared to weak acids, which only partially dissociate.
- Hydrochloric acid, being a strong acid, will produce a higher conductivity compared to ethanoic acid, which is a weak acid.
Experimental Setup:
- Two conductivity meters or conductivity probes can be used to measure the conductivity of solutions of hydrochloric acid and ethanoic acid.
- The solutions are prepared with the same concentration of each acid.
- The conductivity of each solution is measured using the conductivity meter or probe.
Observation:
- Hydrochloric acid, being a strong acid, will show a higher conductivity due to the higher concentration of ions in solution resulting from complete dissociation.
- Ethanoic acid, being a weak acid, will show a lower conductivity due to the lower concentration of ions in solution resulting from partial dissociation.
Interpretation:
- Based on the conductivity measurements, the solution with higher conductivity is identified as hydrochloric acid, while the solution with lower conductivity is identified as ethanoic acid.
Using conductivity measurement in this manner provides a simple and straightforward way to distinguish between strong and weak acids in a school laboratory setting.
Question
(v) Outline how the instrumental method identified in part (a)(iv) distinguishes between the acids in part (a)(iii).
▶️Answer/Explanation
Ans:
The instrumental method identified in part (a)(iv) is conductivity measurement. This method distinguishes between the acids in part (a)(iii) based on their ability to conduct electricity, which is directly related to their degree of dissociation in solution:
1. Hydrochloric Acid (HCl):
- Hydrochloric acid is a strong acid that completely dissociates into ions (\(H^+\) and \(Cl^-\)) in solution.
- The high degree of dissociation results in a high concentration of ions in solution, which greatly enhances the solution’s ability to conduct electricity.
- As a result, hydrochloric acid solutions exhibit high conductivity.
2. Ethanoic Acid (CH3COOH):
- Ethanoic acid is a weak acid that only partially dissociates into ions (\(CH_3COO^-\) and \(H_3O^+\)) in solution.
- The limited dissociation results in a lower concentration of ions in solution compared to hydrochloric acid.
- Consequently, ethanoic acid solutions exhibit lower conductivity compared to hydrochloric acid solutions.
By measuring the conductivity of the solutions using a conductivity meter or probe, we can distinguish between hydrochloric acid and ethanoic acid based on their different conductivities. Hydrochloric acid solutions will show higher conductivity due to their higher concentration of ions resulting from complete dissociation, while ethanoic acid solutions will exhibit lower conductivity due to their lower concentration of ions resulting from partial dissociation. Therefore, conductivity measurement provides a simple and effective way to differentiate between strong acids (like hydrochloric acid) and weak acids (like ethanoic acid) in a laboratory setting.
Question
(b) Outline one chemical test, other than an indicator, that can distinguish between the two acids in part (a)(iii), and the expected result.
Chemical test: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Expected result: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
▶️Answer/Explanation
Ans:
One chemical test that can distinguish between hydrochloric acid (\(HCl\)) and ethanoic acid (\(CH_3COOH\)) is the silver nitrate (\(AgNO_3\)) test.
1. Chemical Test:
- When silver nitrate (\(AgNO_3\)) is added to a solution containing hydrochloric acid (\(HCl\)) and a solution containing ethanoic acid (\(CH_3COOH\)), different reactions occur due to the different behavior of the acids.
2. Expected Result:
Hydrochloric Acid (\(HCl\)):
- When silver nitrate (\(AgNO_3\)) is added to a solution of hydrochloric acid (\(HCl\)), no precipitate forms. This is because hydrochloric acid does not contain any ions that can react with silver ions (\(Ag^+\)).
Ethanoic Acid (\(CH_3COOH\)):
- When silver nitrate (\(AgNO_3\)) is added to a solution of ethanoic acid (\(CH_3COOH\)), a white precipitate of silver ethanoate (\(AgCH_3COO\)) forms. This is because ethanoic acid contains acetate ions (\(CH_3COO^-\)) which can react with silver ions (\(Ag^+\)) to form a precipitate of silver ethanoate.
Therefore, the expected result of the silver nitrate test is:
No precipitate forms when silver nitrate is added to hydrochloric acid.
A white precipitate of silver ethanoate forms when silver nitrate is added to ethanoic acid.
Question
(c) A neutralization curve for a weak acid, HA, and a strong base is given.
(i) Estimate the \(pK_a\) of HA.
▶️Answer/Explanation
Ans:
4.8
Question
(ii) Explain, using an equation, why adding a strong base to the weak acid, HA, leads to very little change in pH in the buffer zone of the graph.
▶️Answer/Explanation
Ans:
When a strong base is added to a weak acid \((H A)\), a buffer solution is formed when the weak acid and its conjugate base \(\left(A^{-}\right)\)are present in approximately equal concentrations. In the buffer zone, the weak acid partially neutralizes the added strong base, resulting in very little change in \(\mathrm{pH}\).
The equilibrium reaction between the weak acid \((H A)\) and its conjugate base \(\left(A^{-}\right)\)is represented as:
\[
\mathrm{HA}+\mathrm{OH}^{-} \rightleftharpoons \mathrm{A}^{-}+\mathrm{H}_2 \mathrm{O}
\]
In this reaction, the hydroxide ions \(\left(\mathrm{OH}^{-}\right)\)from the strong base react with the weak acid \((H A)\) to form water and the conjugate base \(\left(A^{-}\right)\). This reaction helps to neutralize the added base.
In a separate experiment, 80\(cm^3\) of 0.1mol \(dm^{-3}\) ammonia, \(NH_3\) (aq), was added to 40\(cm^3\) of 0.1mol \(dm^{-3}\) hydrochloric acid, HCI(aq).
Question
(iii) Determine the final pH of the solution. Use section 21 of the data booklet.
▶️Answer/Explanation
Ans:
Initial moles of ammonia, \(n(NH_3)_{init} = 0.008\) mol
Initial moles of hydrochloric acid, \(n(HCl)_{init} = 0.004\) mol
Final moles of ammonia, \(n(NH_3)_{fin} = 0.004\) mol
Final moles of ammonium ions, \(n(NH_4^+)_{fin} = 0.004\) mol
Total volume of the resulting solution, \(V_{fin} = 0.12\) dm³
We have already calculated the final concentrations of \(NH_3\) and \(NH_4^+\) to be \(0.033\) mol \(dm^{-3}\).
Now, let’s proceed to calculate the pH of the solution using the \(pK_a\) of \(NH_4^+\):
\[ pK_a(NH_4^+) = 14 – pK_b(NH_3) = 14 – 4.75 = 9.25 \]
Since the solution contains \(NH_4^+\) ions derived from the weak base \(NH_3\), we’ll use the \(pK_a\) value of \(NH_4^+\) to calculate the pH.
The relationship between \(pH\) and \(pK_a\) for a weak acid (\(HA\)) and its conjugate base (\(A^-\)) is given by:
\[ pH = pK_a + \log\left(\frac{[\text{A}^-]}{[\text{HA}]}\right) \]
Since \(NH_4^+\) is the conjugate acid of \(NH_3\), \([\text{NH}_4^+] = [\text{HA}]\), and \([\text{NH}_3] = [\text{A}^-]\).
\[ pH = pK_a + \log\left(\frac{[\text{NH}_3]}{[\text{NH}_4^+]}\right) \]
\[ pH = 9.25 + \log\left(\frac{0.033}{0.033}\right) \]
\[ pH = 9.25 + \log(1) \]
\[ pH = 9.25 \]
Therefore, the final pH of the solution is \(9.25\).