$pH$ of acids, bases and salts

Exercise 5

    

Using the table of acid-base couples, calculate the pH of a methanoic acid solution with concentration c = 0.749 M

Weak acid Let's take $y=[H_3O^+]$ The equation $y^2$ $+$ $K_a\;y$ $-$ $K_a\;c$ $=$ $0$ becomes: $y^2$ $+$ $10^{-3.75}y$ $-$ $10^{-3.75} 0.749$ $=$ $0$ and produces: $y$ $=$ 1.1 10^-2 and so: $pH$ $=$ $ -log\; y$ $ =$ 1.94