Verified answer

HClO = H+ + ClO-

Initial Concentration

HClO H+ Cl-

0.33M.....x.....x

Equilibrium

0.33-x .......+x............+x

Ka = ([H+] * [ClO-]) / [HClO]

3.5e-8 = x^2 / .33-x

(0.33-x) *(3.5e-8) = x^2

1.155e-8 - 3.5e-8x - x^2 = 0

-x^2 - 3.5e-8x + 1.155e-8 =0

Do the quadratic formula

x = 0.00010745

pH = -log[0.00010745]

pH = 3.97