Compared to a weak acid, a strong acid has what pKa value?

Compared to a weak acid, a strong acid has what pKa value?

• A. Lower
• B. Higher
• C. Similar
• D. Undefined

Answer: (A)

The main idea is how pKa relates to acid strength. pKa is the negative logarithm of Ka, so a larger Ka (stronger acid) gives a smaller pKa. Weak acids have small Ka and thus higher pKa. Since a strong acid dissociates almost completely, its Ka is large and its pKa is typically very low, often negative. So, compared to a weak acid, a strong acid has a lower pKa value. For context, common strong acids can have negative pKa values, while weak acids like acetic acid have pKa around 4–5, illustrating the clear difference. Undefined isn’t correct here because pKa is defined for strong acids as well, even if the numeric value is negative.

The main idea is how pKa relates to acid strength. pKa is the negative logarithm of Ka, so a larger Ka (stronger acid) gives a smaller pKa. Weak acids have small Ka and thus higher pKa. Since a strong acid dissociates almost completely, its Ka is large and its pKa is typically very low, often negative. So, compared to a weak acid, a strong acid has a lower pKa value. For context, common strong acids can have negative pKa values, while weak acids like acetic acid have pKa around 4–5, illustrating the clear difference. Undefined isn’t correct here because pKa is defined for strong acids as well, even if the numeric value is negative.