HX(aq) + Y−(aq) ⇄ HY(aq) + X−(aq) Keq > 1 A solution of a salt of a weak acid HY is added to a solut — Acids and Bases Chemistry Question

STEPS:

1. Identify the chemical species and their roles in the equilibrium system:
Look at the provided reversible reaction equation:
$$\text{HX}(aq) + \text{Y}^-(aq) \rightleftharpoons \text{HY}(aq) + \text{X}^-(aq) \quad$$
* $\text{HX}$ acts as the Brønsted^-Lowry acid (proton donor) on the reactant side.
* $\text{Y}^-$ acts as the Brønsted^-Lowry base (proton acceptor) on the reactant side.
* $\text{HY}$ acts as the conjugate acid (proton donor) on the product side.
* $\text{X}^-$ acts as the conjugate base (proton acceptor) on the product side.
2. Analyze the thermodynamic equilibrium constant ($K_{eq}$):
We are given that $K_{eq} > 1$. A value of $K_{eq}$ greater than 1 indicates that the forward reaction is thermodynamically favored, meaning that at equilibrium, the products ($\text{HY}$ and $\text{X}^-$) exist in higher concentrations than the reactants ($\text{HX}$ and $\text{Y}^-$).
3. Apply the principle of competitive acid-base strength:
An acid-base equilibrium always shifts to favor the side containing the weaker acid and weaker base because the stronger acid and stronger base react more effectively to neutralize each other. Since the products are favored ($K_{eq} > 1$), the reactants must be stronger than their corresponding products:
* The reactant acid ($\text{HX}$) is stronger than the product conjugate acid ($\text{HY}$): $\text{HX} > \text{HY}$ in acid strength.
* The reactant base ($\text{Y}^-$) is stronger than the product conjugate base ($\text{X}^-$): $\text{Y}^- > \text{X}^-$ in base strength.
4. Compare the base strengths directly:
The two bases in this reaction are $\text{Y}^-$ and $\text{X}^-$. Since the equilibrium lies to the right, the base $\text{Y}^-$ is more effective at accepting protons than $\text{X}^-$. This mathematically and conceptually establishes that $\text{Y}^-(aq)$ is a stronger base than $\text{X}^-(aq)$.
5. Conclude which species is the strongest base overall:
Because $\text{Y}^-$ is a stronger base than $\text{X}^-$, and because $\text{HX}$ and $\text{HY}$ act as proton-donating acids rather than bases in this system, $\text{Y}^-(aq)$ is the strongest base among all the listed species, making Option B the correct answer.

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WHY_OTHERS_WRONG:

• Option A is incorrect (HX): $\text{HX}$ acts as a Brønsted^-Lowry acid in this reaction, not a base. In fact, because it is the strongest acid in this system, it has the greatest tendency to donate protons and the least tendency to accept them.
• Option C is incorrect (HY): $\text{HY}$ acts as a Brønsted^-Lowry acid on the product side of the reaction, behaving as a proton donor in the reverse direction rather than a base.
• Option D is incorrect (X−): While $\text{X}^-$ is indeed a base (the conjugate base of $\text{HX}$), it is a weaker base than $\text{Y}^-$. Because $\text{HX}$ is a stronger acid than $\text{HY}$, its corresponding conjugate base ($\text{X}^-$) must hold its proton more weakly and be a weaker base than the conjugate base of $\text{HY}$ ($\text{Y}^-$).