Define Ka and pKa and explain how they relate to acid strength.

• A. pKa is the acidity constant; larger pKa means stronger acid.
• B. Ka is the acidity constant; larger Ka means stronger acid. pKa = -log Ka; smaller pKa indicates stronger acid.
• C. Ka is the base dissociation constant.
• D. pKa measures the pH directly.

Answer: (B)

Acid strength is about how readily an acid donates a proton in solution. The key quantity is the acid dissociation constant, Ka, for the equilibrium HA ⇌ H+ + A−, which is defined as Ka = [H+][A−]/[HA]. A larger Ka means the equilibrium lies more to the right, more HA dissociates, and the acid is stronger.

pKa translates this into a convenient scale: pKa = -log10(Ka). Because of the logarithm, a larger Ka gives a smaller pKa. So a stronger acid has both a larger Ka and a smaller pKa. This is why pKa is often used to compare acidity.

What the options convey: Ka being the acidity constant and larger Ka indicating a stronger acid is correct. The relation pKa = -log Ka, and the fact that smaller pKa indicates stronger acid, are also correct. The other statements misidentify Ka as a base dissociation constant, or claim pKa measures pH directly, or say that larger pKa means a stronger acid, which is the opposite of the truth.