Answer
Answer: pH of resulting solution is 7.4
Work Step by Step
*First use Henderson-Hasselbalch equation to calculate the ratio of conjugate base to acid in the starting solution:
pH = pKa + log ([A-]/[HA])
8.0 = 7.4 + log ([A-]/[HA])
0.6 = log ([A-]/[HA])
[A-]/[HA] = 10^0.6 = 4
*Solution contains 100 meq of compound (conjugate base plus acid), thus 80 meq is in conjugate base form and 20 meq is in the acid form, with a [base]/[acid] ratio of 4.
*HCl is a strong acid and dissociates completely. Thus, adding 30 mL of 1.0 M HCl will add 30 meq of H+ to the solution. These new 30 meq titrate 30 meq of the conjugate base, resulting in a [base]/[acid] ratio of 1.
*Last, solve the Henderson-Hasselbalch equation for pH (given pKa 7.4):
pH = pKa + log ([A-/[HA]) = 7.4 + log (1) = 7.4 + 0 = 7.4
Answer: pH of resulting solution is 7.4
