# Making a Buffer Solution

I have worked through the problem and spent quite a bit of time on it but am not extremely confident in my solution. I would just like someone to check my work. Cheers.

## Homework Statement

-Need 0.4 L of an aqueous buffer solution of pH=6.7
-Have access to 0.02 M HCl, 0.02 M NaOH, 0.01 M C3H4N2, and 0.01 M C3H5N2Cl
-Will use Imidazole (C3H4N2) as weak base. Will use C3H5N2Cl as conjugate acid.
-Given Kb for C3H4N2 = 9.0 x 10^-8
-The buffer must have the capacity to absorb 20 mL of either 0.02 M HCl or 0.02 M NaOH and undergo a pH change of no more than +/- 0.1.

## Homework Equations

Henderson-Hasselbach

## The Attempt at a Solution

(Ka)(Kb) = Kw = 1 x 10-14
Ka * 9.0*10^-8 = 1 * 10^-14
Ka = 1.1 x 10^-7

-log(1.1 x 10^-7) = pKa ~ pH = 6.95

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

6.7 = 6.95 + log([base]/[acid])

[base] / [acid] = 10^-0.25 = 0.56 (reasonably close to 1)

Use 1.56 x 10^-5 M for [base] and 0.001 M for [acid]

Buffer concentration = 1.02 x 10^-3

#mol weak base = (buffer concentration) * (desired final volume) = (1.02 x 10^-3) * 0.4 L = 4.06 x 10^-4 mol

0.01 M = 4.06 x 10^-4 mol / ? L

4.06 x 10^-2 L = 40.6 mL C3H4N2

In a 400 mL beaker, add distilled water to 40.6 mL C3H4N2 until most of the desired final volume is reached. Add HCl or NaOH dropwise until the pH reaches 6.7. Add distilled water to the 400 mL line.

## The Attempt at a Solution

Related Biology and Chemistry Homework Help News on Phys.org
Ummm..

:zzz:

This is my last, desperate call for help. Surely someone can lend a hand?

symbolipoint
Homework Helper