Question

Part A Calculate the pH of a buffer that is 0.135 M in NaHC O 3 and 0.180 M in N a 2 C O 3 . Express your answer to two decimal places. Part B Calculate the pH of a solution formed by mixing 65 mL of a solution that is 0.31 M in NaHC O 3 with 75 mL of a solution that is 0.28 M in N a 2 C O 3 . Express your answer to two decimal places.

Answer 1

Part a

Dissociation constant for HCO3-

Ka = 4.67 x 10^-11

pKa = - log (Ka) = - log (4.67*10^-11) = 10.33

From the Henderson-Hasselbalch equation

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

pH = pKa + log [CO3 2-]/[HCO3-]

= 10.33 + log (0.180/0.135)

= 10.46

Part b

Total volume = 65 + 75 = 140 mL

Concentration of Na2CO3 = molarity x volume / total volume

= 0.28 x 75 / 140

= 0.150 M

Concentration of NaHCO3 = 0.31 x 65 / 140 = 0.144 M

[CO3^2-] = 0.150 M

[HCO3-] = 0.144 M

pH = pKa + log [CO3 2-]/[HCO3-]

pH = 10.33 + log (0.150/0.144)

= 10.35