Which statement is true about Ka and [H+]?

• A. Ka increases as [H+] increases
• B. Ka is independent of [H+] and depends mainly on temperature
• C. Ka decreases as [H+] increases
• D. Ka equals [H+][A-]/[HA] regardless of temperature

Answer: (B)

Ka is an equilibrium constant for the dissociation of a weak acid, and for a given temperature its value stays fixed. It is defined by Ka = [H+][A-]/[HA], so the ratio depends on the equilibrium concentrations at that temperature, not on how much hydrogen ion is present overall in the solution at that moment. If you add more H+ (for example, by using a strong acid), the position of the equilibrium shifts to reduce dissociation, changing the individual concentrations, but Ka itself does not change unless the temperature changes. This is why the statement that Ka is independent of [H+] and mainly depends on temperature is the correct one. The idea that Ka changes with [H+] or that Ka remains the same regardless of temperature ignores how the value of Ka is set by temperature in the first place.