KOH is a Brønsted-Lowry base because
A) it is a polar molecule
B) it can dissolve in water
C) it is a hydroxide donor
D) it is a proton acceptor
The Correct Answer and Explanation is:
The correct answer is D) it is a proton acceptor.
Under the Brønsted-Lowry acid-base theory, acids are substances that can donate protons (H⁺ ions), while bases are substances that can accept protons. Potassium hydroxide (KOH) is classified as a Brønsted-Lowry base due to its ability to accept protons, which corresponds to answer choice D.
When KOH dissolves in water, it dissociates completely into potassium ions (K⁺) and hydroxide ions (OH⁻). The hydroxide ion (OH⁻) is highly reactive and has a strong affinity for hydrogen ions (protons). In the presence of an acid (proton donor), the hydroxide ion (OH⁻) readily accepts a proton to form water (H₂O):
[
\text{OH}^- + \text{H}^+ \rightarrow \text{H}_2\text{O}
]
This reaction exemplifies the Brønsted-Lowry definition because the hydroxide ion, a component of KOH, functions as a proton acceptor.
The other answer choices don’t accurately reflect why KOH is considered a Brønsted-Lowry base. While KOH is indeed a polar molecule (choice A), this characteristic is related more to the solubility of KOH in water rather than its ability to accept protons. Its polarity enables it to dissociate in water, but it does not inherently define its acidic or basic nature. Choice B, KOH’s ability to dissolve in water, is also correct but irrelevant to its role as a Brønsted-Lowry base. Many substances dissolve in water without acting as bases or proton acceptors. Choice C, hydroxide donor, is incorrect in this context; while KOH releases hydroxide ions, it is the hydroxide ion’s role as a proton acceptor, not its release from KOH, that defines KOH’s behavior as a base under the Brønsted-Lowry theory.
In summary, KOH is classified as a Brønsted-Lowry base because the hydroxide ion it releases can accept a proton from an acid, forming water. This proton acceptance is the defining behavior of a Brønsted-Lowry base.