Consider two aqueous solutions of nitrous acid (HNO2, Ka = 4.6 × 10–4. Solution A has a concentration of [HNO2] = 0.55 M and solution B has a concentration of [HNO2] = 1.25 M. Which statement is true?

Question

BlackTom AI · Accepted Answer

When evaluating a weak acid like HNO2, two key relationships matter: (1) percent ionization tends to be larger for lower starting concentrations, and (2) the pH depends on the actual [H+] produced, which for a weak acid roughly scales with sqrt(Ka × C).

Option 1: 'Solution B has a higher percent ionization and higher pH.' For percent ionization, B has the stronger initial concentration (1.25 M) compared with A (0.55 M), so B will actually have a smaller percent ionization, not larger. Regarding pH, a larger concentration tends to produce more H+, giving a lower pH; thus B would not have a higher pH than A. This option is inconsistent with the expected trend for both metrics.

Option 2: 'Solution A has a higher percent ionization and higher pH.' This aligns with the general rule that a weaker initial concentration yields higher percent ionization for a given Ka, since percent ionization ≈ sqrt(Ka / C). Because A has the smaller concentration, its percent ionization is higher. Additionally, with a smaller C, the produced [H+] is smaller, leading to a higher pH (less acidic) compared with B. This option matches both observed trends for a weak acid across differing concentrations.

Option 3: 'Solution B has a higher percent ionization and solution A has a higher pH.' As discussed, B does not have the higher percent ionization; in fact, B’s percent ionization should be lower due to its larger initial concentration. The second part about A having the higher pH is true, but since the first claim is incorrect, the overall statement is flawed.

Option 4: 'Solution A has a higher percent ionization and solution B has a higher pH.' The first half is correct (A would have higher percent ionization), but the second half is wrong: B would not have a higher pH than A; B’s higher [H+] from the larger concentration makes its pH lower. Hence this option misstates the pH relationship.

In summary, evaluating both the percent ionization behavior and the pH implications for solutions A and B shows that the most consistent statement with weak-acid chemistry is the one asserting higher percent ionization and higher pH for the lower concentration (A).

Consider two aqueous solutions of nitrous acid (HNO2, Ka = 4.6 × 10–4. Solution A has a concentration of [HNO2] = 0.55 M and solution B has a concentration of [HNO2] = 1.25 M. Which statement is true?

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