Calculate pH after addition of mL

Question

Calculate the pH after the addition of 16.31 mL of KOH in the titration of 25.00 mL of 0.20 M HOCl with 0.10 M KOH. Ka for HOCl = 3.0 x 10-8

Answer 1

find moles:

0.025 litres @ 0.20 mol/litre = 0.005000 moles acid

0.01631 litres @ 0.1 mol/litre = 0.001631 moles of base

when these react 1:1,  you will only have 0.003369 moles of acid left,

but you will have made 0.001631 moles of NaClO

find molarities:

0.003369 moles / 0.04131 litres = 0.08155 Molar acid

0.001631 moles / 0.04131 litres = 0.03948 Molar ClO-

HClO --> H+ & ClO-

K = [H+] [ClO-] / [HClO]

3.0 x 10-8 = [H+] [0.03948] / [0.08155]

H+ = 6.20e-8

your answer is: pH = 7.21