Question

Calculate the pH of a buffer that is 0.300 M in NaHCO3 and 0.325 M in Na2CO3.

Express your answer using two decimal places

Calculate the pH of a solution formed by mixing 65 mL of 0.35 M NaHCO3 with 75 mL of 0.30 M Na2CO3.

Answer 1

For HCO₃^- , K_a = 5.6 *10^-11

pKa = -logKa = -log (5.6 *10^-11) = 10.25

(a) [Na₂CO₃] = 0.325 M

and [NaHCO₃] = 0.300 M

From Henderson Hasselbach equation

pH = pKa + log [Na₂CO₃] / [NaHCO₃] = 10.25 + log (0.325/0.300) = 10.29

(b) Total volume = 65 + 75 = 140 mL

[Na₂CO₃] in solution = Initial molarity * volume / Total volume = 75 mL *0.30 / 140 mL = 0.16 M

[NaHCO₃] = 65mL * 0.35 M / 140mL = 0.1625 M

pH = 10.25 +log (0.16/0.1625) = 10.247 = 10.25 (approx)