$pH$ of acids, bases and salts

Exercise 5

    

Using the table of acid-base couples, calculate the pH of a cyanhydric acid solution with concentration c = 0.206 M

Weak acid Let's take $y=[H_3O^+]$ The equation $y^2$ $+$ $K_a\;y$ $-$ $K_a\;c$ $=$ $0$ becomes: $y^2$ $+$ $10^{-9.31}y$ $-$ $10^{-9.31} 0.206$ $=$ $0$ and produces: $y$ $=$ 1 10^-5 and so: $pH$ $=$ $ -log\; y$ $ =$ 5