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Gaseous HCl was bubbled into water and two solutions, X and Y - HSC - SSCE Chemistry - Question 34 - 2021 - Paper 1
Question 34
Gaseous HCl was bubbled into water and two solutions, X and Y. Solutions X and Y contain the same type of ions. The pH of each was monitored over time and recorded i... show full transcript
Worked Solution & Example Answer:Gaseous HCl was bubbled into water and two solutions, X and Y - HSC - SSCE Chemistry - Question 34 - 2021 - Paper 1
Step 1
Explain the pH of water at t₀, t₁ and t₂
Answer
At t₀, the pH of the water is 7, indicating a neutral solution.
At t₁, the pH of water drops due to the introduction of HCl gas, which dissociates in water to produce hydronium ions, resulting in a decrease in pH as follows:
This reaction explains the rapid decrease we see in the pH. The H⁺ ions in the solution increase the acidity, lowering the pH rapidly.
At t₂, the pH of the water stabilizes, although it remains acidic. The water can only buffer to a certain extent but cannot neutralize all the H⁺ ions, leading to a stabilized pH that is still below 7.
Step 2
Explain the pH of solution X at t₀, t₁ and t₂
Answer
At t₀, the pH of solution X is higher than that of the water due to the presence of a weaker acid or a buffer, allowing it to maintain a more basic pH initially.
At t₁, we see a rapid drop in pH similar to the water, but less steeply. This indicates buffer action as the acid interacts with the components in the solution to minimize changes in pH. The introduction of H⁺ ions from the HCl gas reacts with the buffer components rather than completely overwhelming the buffer, thus reducing the impact of the acid.
At t₂, solution X's pH decreases further but remains above pH 4.9, indicating that the buffering components are still partially effective but are becoming exhausted, leading to a gradual decrease in pH.
Step 3
Explain the pH of solution Y at t₀, t₁ and t₂
Answer
At t₀, the pH of solution Y is slightly acidic, likely indicating a less effective buffer system compared to solution X.
At t₁, the pH drops more sharply than that of solution X, reflecting that it has less buffering capacity, leading to a faster increase in H⁺ concentration from the HCl.
By t₂, the pH of solution Y stabilizes at a relatively low level (below 4.9), indicating that the buffering action has been overwhelmed, and the solution has become more acidic overall. The capacity to neutralize incoming H⁺ ions has been greatly reduced.