$pH$ of acids, bases and salts
Exercise 5

Using the table of acid-base couples, calculate the pH of a hypochlorous acid solution with concentration c = 0.549 M
Weak acid
Let's take $y=[H_3O^+]$
The equation
$y^2$ $+$ $K_a\;y$ $-$ $K_a\;c$ $=$ $0$
becomes:
$y^2$ $+$ $10^{-7.3}y$ $-$ $10^{-7.3} 0.549$ $=$ $0$
and produces:
$y$ $=$ 1.7 10-4
and so:
$pH$ $=$ $ -log\; y$ $ =$ 3.78