If the acid dissociation constant Ka increases, which statement correctly describes pKa and acid strength?

If the acid dissociation constant Ka increases, which statement correctly describes pKa and acid strength?

• A. pKa decreases and acid becomes stronger
• B. pKa remains unchanged
• C. pKa increases and acid becomes stronger
• D. pKa decreases and acid becomes weaker

Answer: (A)

The key idea is how Ka and pKa relate to acid strength. Ka measures how much an acid dissociates in water: a larger Ka means the equilibrium favors the right side, so the acid is stronger. pKa is defined as the negative logarithm of Ka, pKa = -log10(Ka). When Ka increases, log10(Ka) increases, and taking the negative turns that into a smaller number. So a bigger Ka corresponds to a smaller pKa, meaning a stronger acid. For instance, if Ka goes from 1×10^-5 to 1×10^-3, pKa drops from about 5 to 3, reflecting increased acidity. Therefore, describing pKa as decreasing and the acid becoming stronger matches how Ka and pKa relate. The other ideas—pKa staying the same or increasing, or the acid becoming weaker—don’t fit because they contradict the inverse relationship between Ka and pKa and what larger Ka means for acid strength.

The key idea is how Ka and pKa relate to acid strength. Ka measures how much an acid dissociates in water: a larger Ka means the equilibrium favors the right side, so the acid is stronger. pKa is defined as the negative logarithm of Ka, pKa = -log10(Ka). When Ka increases, log10(Ka) increases, and taking the negative turns that into a smaller number. So a bigger Ka corresponds to a smaller pKa, meaning a stronger acid. For instance, if Ka goes from 1×10^-5 to 1×10^-3, pKa drops from about 5 to 3, reflecting increased acidity. Therefore, describing pKa as decreasing and the acid becoming stronger matches how Ka and pKa relate. The other ideas—pKa staying the same or increasing, or the acid becoming weaker—don’t fit because they contradict the inverse relationship between Ka and pKa and what larger Ka means for acid strength.