A buffer prepared by dissolving oxalic acid dihydrate (H2C2O4⋅2H2O) and disodium oxalate (Na2C2O4) in 1.00 L of water has a pH of 4.619. How many grams of oxalic acid dihydrate (MW=126.07 g/mol) and disodium oxalate (MW=133.99 g/mol) were required to prepare this buffer if the total oxalate concentration is 0.150 M? Oxalic acid has pKa values of 1.250 (pKa1) and 4.266 (pKa2).

There is needed 5.8 g of NaHC2O4 and 13.89 g of Na2C2O4

Explanation:

Buffer solution is a combination of a weak acid and its conjugate base or a weak base and its conjugate acid.

Step 1: The equation

H2C2O4 + NaOH ⇔ NaHC2O4 + OH - pKa1 = 1.250

NaHC2O4 + NaOH ⇔ Na2C2O4 + OH - pKa2 = 4.266

Step 2: The pH equation

pH = pKa + log [conjugate base]/[acid]

pH = pKa + log [Na2C2O4]/[NaHC2O4]

4.619 = 4.266 + log [Na2C2O4]/[NaHC2O4]

0.353 = log [Na2C2O4]/[NaHC2O4]

10^0353 = [Na2C2O4]/[NaHC2O4]

2.25424 = [Na2C2O4]/[NaHC2O4]

[Na2C2O4] = 2.25424 * [NaHC2O4]

Step 3: Calculate the concentrations

Given is that [Na2C2O4] + [NaHC2O4] = 0.150M

So 2.25424 * [NaHC2O4] + [NaHC2O4] = 0.150M

[NaHC2O4] = 0.046 M

[Na2C2O4] = 2.25424 * [NaHC2O4] = 2.25424 * 0.046 M = 0.1037 M

Step 4: Calculate mass

0.046 moles NaHC2O4 * 126.07g/mole = 5.8 g

0.1037 moles Na2C2O4 x 133.99g/mole = 13.89 g