Question

You will prepare 50.0 mL of a pH = 5.0 acetic acid/sodium acetate buffer solution. You will use 10.0 mL of 0.100 M acetic acid. You will need to determine the amount of sodium acetate (the conjugate base) to use to prepare the buffer.

How do I determine the amount of sodium acetate needed for my buffer?

Answer 1

Use the HH equation:
pH = pKa + log[A^-]/[HA]

From there, solve for the concentration of A^- and then, calculate moles, and finally the mass with the molecular weight:

moles of acid = 0.1 * 0.10 = 0.001 moles
[HA] = 0.001 / 0.050 = 0.02 M

pKa of acetic acid = 4.76
Molecular weight of sodium acetate = 82.03 g/mol

5 = 4.76 + log[A^-]/0.02
10^(5-4.76) = [A^-] / 0.02
[A^-] = 0.0347 M
moles A^- = 0.0347 * 0.050 = 0.00174 moles
mass A^- = 0.00174 * 82.03 = 0.1427 g

Hope this helps