Question

C. You wish to prepare 100 ml (total volume) of a buffer solution that is 0.025 m in carbonic acid, ph 7.25. You have solid h2co3 and solid nahco3. Calculate how many grams of h2co3 and hco3- you need to weigh out to prepare this solution. Mass of h2co3 _________ g and mass of hco3- _________ g

Answer 1

Total volume required = 100mL

The concentration of carbonic acid = 0.025

Let concentration of sodium bi carbonate = [NaHCO3]

From Hendersen Hassalbalch's equation for buffers

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

pKa of carbonic acid = 6.35

7.25 = 6.35 + log [NaHCO3] / 0.025

0.9 = log [NaHCO3] / 0.025

Taking antilog

7.943 =  [NaHCO3] / 0.025

[NaHCO3]  = 7.943 X  0.025

[NaHCO3]  = 0.199 M

Volume is 100mL

Moles of NaHCO3 = molarity X volume (L) = 0.199 X 0.1 = 0.0199

Mass of NaHCO3 = moles X molar mass = 0.0199 X 84 = 1.672 g

moles of H2CO3 = molarity X volume (L) = 0.025 X 0.1 = 0.0025

Mass of H2CO3 = moles X molar mass = 0.0025 X 62 = 0.155 g