Question

Previous question: To 100 mL of pure H2O, you add 0.01 moles of the weak acid KHP (potassium hydrogen phthalate). Assuming the system behaves ideally (activity coefficients are all equal to 1), calculate and report the pH of the resulting solution. ANSWER: pH=3.2

Still considering the KHP system from the previous question, let’s now consider the non-ideal system in which the activity coefficients are γKHP = 0.95, γKP -= 1.2, γH3O+ = 0.95. In terms of absolute concentrations, what is the effect of these activity coefficients on the extent of this reaction? Or the apparent strength of this acid? What is the effective pKA under these conditions?

Answer 1

Molarity of 0.01 moles= 0.710 grams KHP

Molarity = mass/Molar mass of the KHP (204.24 g/mol) = 0.710/ 204.24= 0.00348M

pH = - log (activity coefficient x concentration of acid)

   = - log ( 1 x 0.00348)

= -log 0.00348

    = 2.45

For activity coefficient 0.95,1.2, 0.95

pH₂= - log (0.95 x 0.00348) = 2.48

pH₂= - log (1.2 x 0.00348) = 2.45

As the activity coefficient decreases pH increases

For weak acid pKa = 1/2 pH

therefore pKa= 2.45/2 = 1.22