pH is the scale of measurement to check either the solution is acidic or basic. if the pH value is greater than 7, the solution is towards basicity and if the pH value is less than 7, the solution is towards acidity.
pKa is the dissociation constant and it is used to describe the change in pH value, when pKa is equal to pH: 50% of the ionizable group is protonated and 50% of the enzymes will be active.
So 50% of the enzymes will be active.