When the concentration of a weak acid in solution is equal to the concentration of its conjugate base in solution, then:
Would pH=pKa or would it equal pH=pKa + 1 like Henderson-Hasselbalch equation?
When the concentration of a weak acid in solution is equal to the concentration of its conjugate base in solution, the pH of the solution would be equal to the pKa of the weak acid. This is because the pH of a solution is directly related to the concentration of the acid and its conjugate base according to the Henderson-Hasselbalch equation.
The Henderson-Hasselbalch equation is given by:
pH = pKa + log([A-]/[HA])
where pH is the acidity of the solution, pKa is the acid dissociation constant of the weak acid, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.
In the scenario you described, when [A-] is equal to [HA], the logarithmic term becomes equal to zero. Therefore, the pH of the solution will be equal to the pKa of the weak acid.
So, in conclusion, when the concentration of a weak acid in solution is equal to the concentration of its conjugate base in solution, the pH of the solution will be equal to the pKa of the weak acid, without any additional adjustment of "+1" as in the Henderson-Hasselbalch equation.