Question

Determine the [H3O*1 and pH of a 0.200 M formic acid (HCHO2) solution. The Ka of formic acid at 25 °C is 1.8 X 10. (1 point)

Answer 1

HCHO2 + H2O ----> H3O+ + HCO2-

initially 0.2 M 0 0

equilibrium 0.2-x x x

Ka = [ H3O+] * [HCO2-] / [HCHO2]

1.8 * 10^-4 = x *x / ( 0.2 - x)

1.8*10^-4 = x^2 / ( 0.2 - x)

x is very less compared to 0.2

( 0.2 - x) ~ 0.2

1.8*10^-4 = x^2 / 0.2

x^2 = 0.2*1.8*10^-4

x^2 = 0.36*10^-4

x = 0.6*10^-2

x = 0.006 M

so the concentration of [H3O+] = 0.006 M

pH = - log [H3O+]

pH = - log ( 0.006 )

pH = 2.22

pH of the solution is = 2.22