To determine which of the given substances best models a weak acid in an aqueous solution, we need to understand the characteristics of weak acids and the nature of each substance:
### Characteristics of a Weak Acid:
1. Incomplete Dissociation: A weak acid does not fully dissociate into its ions in water.
2. Equilibrium: In an aqueous solution, it reaches an equilibrium between the undissociated acid and its ions.
Now, let's analyze each option:
1. HCl (Hydrochloric Acid):
- HCl is a strong acid, meaning it completely dissociates in water into [tex]\( \text{H}^+ \)[/tex] and [tex]\( \text{Cl}^- \)[/tex] ions.
- Therefore, HCl is not a weak acid.
2. NaF (Sodium Fluoride):
- NaF is a salt formed from the neutralization of a strong base (NaOH) and a weak acid (HF).
- When dissolved in water, NaF dissociates completely into [tex]\( \text{Na}^+ \)[/tex] and [tex]\( \text{F}^- \)[/tex] ions.
- NaF itself is not an acid, so it doesn't fit the criteria.
3. Cr (Chromium):
- Chromium (Cr) is a metal element and not an acid.
- It does not dissociate in water to form an acidic solution.
4. HF (Hydrofluoric Acid):
- HF is a weak acid because it does not completely dissociate in water.
- In an aqueous solution, the equilibrium is established as follows:
[tex]\[
\text{HF} \rightleftharpoons \text{H}^+ + \text{F}^-
\][/tex]
- Only a fraction of HF molecules dissociate to form [tex]\( \text{H}^+ \)[/tex] and [tex]\( \text{F}^- \)[/tex] ions.
### Conclusion:
Among the given options, HF (Hydrofluoric Acid) best models a weak acid in an aqueous solution because it only partially dissociates in water, fitting the definition of a weak acid.
So, the correct answer is HF.
