What Is Ka? The Acid Dissociation Constant Explained
"Strong" and "weak" are useful words right up to the moment someone asks how weak. Is hydrofluoric acid weaker than formic acid? By how much? That is what Ka is for: it turns a vague adjective into a number you can calculate with. The short answer: Ka , the acid dissociation constant , is the equilibrium constant for an acid giving up its proton to water. A larger Ka means a stronger acid — at the same concentration, more of it is ionised at equilibrium — and because the numbers are tiny, they are usually quoted as pKa = −log Ka , where a smaller pKa means a stronger acid . What Ka actually is Every weak acid in water sets up an equilibrium: HA + H₂O ⇌ H₃O⁺ + A⁻ Write the equilibrium expression for it and you get Ka: Ka = [H₃O⁺][A⁻] ÷ [HA] Water does not appear, even though it is a reactant. It is the solvent, present in enormous excess, and its concentration barely changes — so by convention it is folded into the constant rather than written out. Read th...