Question

Calculate the acetic acid and acetate ion concentrations in a 10mL, 1M solution with a pH of 2.59. The Ka is 6.78x10^-6. Also determine the experimental value of Ka for this experiment.

Answer 1

Suppose concentration of acetic acid = x M

Then Concentration of acetate ion is = (1-x) M

Ka = 6.78 * 10^-6

pKa = - log Ka = - log ( 6.78 * 10^-6) = - log 6.78 + 6 log 10

pKa = -0.83 + 6*1 = 5.17

We know from Henderson equation

pH = pKa + log {[acetate ion ] / [acetic acid] }

2.59 = 5.17 + log {(1-x) M / (xM)}

2.59 - 5.17 = log (1-x/ x)

-2.58 = log (1-x/x)

1-x / x = 10^-2.58 = 0.00263

1-x = 0.00263 x

x + 0.00263 x = 1

1.00263 x = 1

x = 1/ 1.00263 = 0.997

Concentration of acetic acid = x M = 0.997 M

Concentration of acetate ion = (1-x)M = (1-0.997)M = 0.003 M