Question

A 21.1 mL sample of a 0.485 M aqueous hypochlorous acid solution is titrated with a 0.347 M aqueous sodium hydroxide solution. What is the pH at the start of the titration, before any sodium hydroxide has been added?

12- A 24.0 mL sample of 0.323 M ethylamine, C2H5NH2, is titrated with 0.284 M nitric acid. At the titration midpoint, the pH is :________________.

Answer 1

There are multiple questions here . i am allowed to answer only 1 at a time. I will answer 1st one for you. Please ask other as different question

Ka of hypochlorous acid acid (HOCl)= 3.5*10^-8

HOCl   <------> H+ + OCl-
0.485-X               X            X

Ka= [H+][OCl-]/ [HOCl]
3.5*10^-8 = X*X / (0.485-X)
Since Ka is very small, X will be small and it can be ignored as compared to 0.485
3.5*10^-8 = X*X / 0.485
X= 1.3*10^-4 M

This is concentration of H+
[H+] = 1.3*10^-4 M

pH = -log [H+]
   = -log (1.3*10^-4)
   = 3.9
Answer: 3.9