Acid Dissociation Constant Ka: Tells us extent of dissociation. No Ka value for strong acid as
completely dissociate. Units are moldm-3.
Ka =
[ H +(aq)][ A−(aq)]
[ HA(aq)]
Use this one if Q asks for Ka expression.
Equilibrium Constant: Ka changes with temperature so Ka standardised at 25C. Larger Ka means
equilibrium further to right, more H+ dissociates so pH decreases.
pKa: Convert Ka to pKa as Ka has negative indices like [H+ (aq)] so pKa easier to compare relative
numbers. The larger the Ka value but smaller the pKa value.
pKa = – logKa
Ka = 10-pKa