Calculate the [H+] and pH of a 0.000173 M acetic acid solution. Keep in mind that the Ka of acetic acid is 1.76×10^−5.

Question

Calculate the [H+] and pH of a 0.000173 M acetic acid solution. Keep in mind that the KA of acetic acid is 1.76×10^−5.

This is a problem that will require you to use the quadratic. Start by setting up the quadratic equation before using the quadratic formula and find the coefficients. Then, solve the quadratic. 

Quadratic formula: ax^2+bx+c=0; enter the values of a, b, c.

Answer 1

Let HAc = acetic acid

HAc ==> H+ + Ac-

Ka = 1.76x10^-5 = [H+][Ac-] / [HAc]

1.76x10^-5 = (x)(x) / 0.000173 - x

1.76x10^-5 = x² / 1.73x10^-4 - x

x² = - 1.76x10^-5x + 3.04x10^-9

x² + 1.76x10^-5x - 3.04x10^-9 = 0

For the quadratic: a = 1, b = 1.76x10^-5 and c = - 3.04x10^-9

Solving for x:

x = 4.70x10^-5 M =[H⁺]

pH = -log [H+] = -log 4.70x10^-5

pH = 4.33