Question

Arsenic acid, H3AsO4 (MW= 141.94 g/mol), is a triprotic acid. The Ka values for arsenic acid are Ka1 = 5.0 x 10^ -4, Ka2 = 8.0 x 10^ -8, and Ka3 = 6.0 x 10 ^ -10. What volumes of 0.25 M H3AsO4, 0.25 M NaOH, and distilled water are required to make a buffer with a pH = 8? The total [AsO4] = 0.05M and the total volume is 1 L.

Answer 1

PKa2 is calculated, since it is the closest to the required pH:

pKa2 = - log 8x10 ^ -8 = 7.1

The molar ratio of the buffer is calculated:

n Salt / Acid = 10 ^ (pH - pKa) = 10 ^ (8 - 7.1) = 8

It has:

1) n Salt - 8 * n Acid = 0

2) n Salt + n Acid = 0.05 * 1 = 0.05

System of equations is applied and you have:

n Salt = 0.044 mol

n Acid = 0.006 mol

The volume of initial acid required is calculated:

V H3AsO4 = 0.025 M * 1000 mL / 0.05 M = 500 mL

Starting from a triprotic acid, the entire first proton must be neutralized:

Vi NaOH = 500 mL

Then the required volume of the second proton is neutralized:

Vii NaOH = n salt * 1000 / M = 0.044 * 1000 / 0.25 = 176 mL

Vt = 676 mL

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