When 35.0 mL of 0.400 M hydrochloric acid and 35.0 mL of 0.400 M sodium hypochlorite are combined, the pH of the resulting solution will be :

pH = 7.49

Explanation:

This is a buffer solution. We can apply Henderson Hasselbach equation:

pH = pKa + log ((total mmoles - mmoles HCl) / mmoles HCl)

We have the same amount of mmoles.

35 mL . 0.400 M = 14 mmoles HCl

35 mL . 0.400 M = 14 mmoles NaClO

Total moles are 28 mmoles.

We replace data:

pH = pKa + log ((28 mmoles -14 mmoles) / 14 mmoles)

Notice that the relation in log = 1. So pH = pKa

pKa for HClO is 7.49

When pH = pKa we have the same amount of base and acid. Notice, that we have the same mmoles of HCl and NaClO.

Remember that a buffer can be prepared with:

a) A weak acid and its conjugate base (acetic /acetate)

A weak base and its conjugate acid (ammonia/ammonium)

b) A weak acid with a strong base, where the acid can be in excess.

c) A conjugate base (salt from weak acid) with a strong acid, where the base can be in excess. (This case but, the base is not in excess, that's why ph = pKa)