pH and pKa: two numbers answering different questions
Distinguish solution acidity from an acid’s dissociation equilibrium, and make the assumptions behind a calculation visible.
Separate substance and solution
pH describes a solution’s acidity state. pKa relates to an acid dissociation equilibrium under specified conditions. Knowing an acid’s pKa does not imply that every solution made from it has the same pH. Concentration, composition and conditions still matter.
Activity and approximation
The definition of pH uses hydrogen-ion activity. Introductory dilute-solution calculations often relate activity to concentration through an approximation. Identify that assumption rather than treating it as universally exact.
Write the equilibrium constant, concentration, units and temperature used. Keep the number of reported digits consistent with the information in the inputs. A long calculator display is not evidence of chemical precision.
Check the method afterwards
Set up the equilibrium and material balance first. If a small-change approximation is used, verify that the resulting change is small enough for the intended approximation. Otherwise use a more appropriate solution.
Before requesting one numerical answer, ask which solution, concentration and conditions are being discussed. Missing context cannot be repaired by adding decimal places.
