Which of the following statements correctly compares a strong acid to a weak acid and describes Ka?

• A. Strong acids completely dissociate; Ka is small.
• B. Weak acids completely dissociate; Ka is large.
• C. Strong acids partially dissociate; Ka is small.
• D. Weak acids partially dissociate; Ka is small.

Answer: (D)

Understanding acid strength and Ka is about how far the acid splits in water. For an acid HA, Ka = [H+][A−]/[HA] measures how much dissociation occurs. Strong acids dissociate almost completely, so they have a very large Ka and little undissociated HA remains. Weak acids only dissociate a small amount, so Ka is small and most of the acid stays as HA. The statement that matches this is: weak acids partially dissociate and Ka is small. It reflects that partial dissociation goes hand in hand with a small Ka, while complete dissociation would correspond to a large Ka.