weak acid pH calculation

Question

H2S (Ka = 1.1 x10 -7 ) is dissolved in water to a concentration of 6.00 g H2S per 100.00 mL of water. Calculate the pH of this solution. Calculate the percent ionization the H2S solution.

Answer 1

Molar mass H2S = 34 g/mol

6.00 g H2S x 1 mol/32g = 0.1875 mol

0.1875 mol/100ml x 1000 ml = 1.875 mol/L

H2S <==> H+ + HS-

Ka = [H+][HS-] / [H2S]

1.1x10^-7 = (x)(x) / 1.875 - x and assume x is small relative to 1.875

1.1x10^-7 = x² / 1.875

x² = 2.1x10^-7

x = [H+] = 4.5x10^-4 M

pH = -log 4.5x10^-4 M

pH = 3.34

% ionization = 4.5x10^-4 / 1.875 x100 = 0.024% ionized