Question

You have been supplied with 2M solution of carbonic acid (H2CO3) and a 6M solution of NaOH. What volumes, of these materials are required to prepare 1 L of a 50 mM buffer solution at pH 7 (pKa for carbonic acid is 6.35)

Answer 1

The volume of H2CO3 solution necessary to prepare said buffer concentration is calculated:

V1 = C2 * V2 / C1 = 0.05 M * 1000 mL / 2 M = 25 mL

The molar ratio of acid and salt is calculated, by means of the equation of henderson hasselbach cleared:

1) n Salt / n Acid = 10 ^ (pH - pKa) = 10 ^ (7 - 6.35) = 4.47

You have:

2) n Salt + n Acid = M * V = 0.05 M * 1 L = 0.05 mol

System of equations between 1 and 2 is applied:

n Salt = 0.041 mol

n Acid = 0.009 mol

The volume of NaOH necessary to produce said amount of salt from the initial acid is calculated:

V NaOH = n Salt * 1000 / M = 0.041 * 1000/6 = 6.83 mL

They are required:

V acid = 25 mL

V NaOH = 6.83 mL

V Water = 68.17 mL

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