Question

A buffer is made by combining monosodium oxalate (NaC2O4H) and disodium oxalate (Na2C2O4) to give concentrations of 0.01 M for NaC2O4H and 0.02 M for Na2C2O4. pKa1 = 1.25 and pKa2 = 4.28

a) What is the initial pH of this buffer?

b) What is the buffer intensity?

c) What is the pH of this buffer after the addition of 0.01 M NaOH?

Answer 1

a)

initially, we must appl y henderson hasselbach equations

use 2nd ionization, sinc eC2O4-2 and HC2O4- are present on this only

pH = pKa2 + log(C2O4-2 / HC2O4-)

pH = 4.28 + log(0.01/0.02)

ph = 3.9789

b)

buffer intensity --> total molaritty

M total = 0.01 + 0.02 = 0.03 M

c)

after

0.01 M o fNaOH

C2O4H + OH- = H2O + C2O4-2

then

[C2O4-2] = 0.01+0.02 = 0.03 M

this can be calculatesd as:

C2O4-2 + H2O <-> HC2O4- + OH- Kb2

Kb2 = Kw/Ka2 = (10^-14)/(10^-4.28) = 1.905*10^-10

Kb = [HC2O4-][OH-]/[C2O4-2]

1.905*10^-10 = x*x/(0.03-x)

x = 2.39*10^-6

[OH-] = 2.39*10^-6

pOH = -log(2.39*10^-6) = 5.62

pH = 14-5.62 = 8.32