pH of acids, bases and salts

Exercise 4

    

Calculate the pH of an ethanoic acid solution ($pK_a=4,75$) with concentration c = 0.271 M

Weak acid Assume $y=[H_3O^+]$ The equation $y^2+K_ay-K_ac=0$ becomes: $y^2 +10^{-4,75}y-10^{-4,75} 0.271=0$ and produces $y=$ 2.2 10^-3 so: $pH= -log\; y =$ 2.66