If Ka is larger, what does this indicate about the acid?

If Ka is larger, what does this indicate about the acid?

• A. The acid is stronger in solution.
• B. The acid is weaker in solution.
• C. The strength depends on concentration.
• D. The acid has no defined strength.

Answer: (A)

Ka shows how far an acid dissociates in water. It’s defined by Ka = [H3O+][A−]/[HA], so a larger Ka means the equilibrium lies more toward the right, producing more hydronium ions and conjugate base for the same starting amount of the acid. That increased dissociation lowers the solution’s pH, which is what we mean when we say the acid is stronger in solution. Since pKa = −log Ka, a larger Ka also gives a smaller pKa, reinforcing the idea that the acid is stronger. Remember, Ka is temperature dependent and, for very strong acids, Ka is very large because dissociation is essentially complete.

Ka shows how far an acid dissociates in water. It’s defined by Ka = [H3O+][A−]/[HA], so a larger Ka means the equilibrium lies more toward the right, producing more hydronium ions and conjugate base for the same starting amount of the acid. That increased dissociation lowers the solution’s pH, which is what we mean when we say the acid is stronger in solution. Since pKa = −log Ka, a larger Ka also gives a smaller pKa, reinforcing the idea that the acid is stronger. Remember, Ka is temperature dependent and, for very strong acids, Ka is very large because dissociation is essentially complete.