You want to make an acetic acid/acetate buffer, pH 4.3, with a volume of 250 mL, and a final concentration of?

Question

([AcOH] + [AcO-]) = 0.1 M. You may only use acetic acid and sodium acetate (no strong acid or base). How many mL of glacial acetic acid would be needed (d = 1.05 g/mL)?

a. ) 1.16 mL

b. ) 1.05 mL

c. ) 0.38 mL

d. ) 2.38 mL

Answer 1

Acetic acid pKa = 4.75, MW = 60 g/mole.

pH = pKa + log([A-]/[AH])

4.3 = 4.75 + log([A-]/[AH])

Solving, [A-]/[AH] = 0.355.

We are given [AH] + [A-] = 0.1

Solving the simultaneous equations,

[AH] = 0.0738 M

[A-] = 0.0262 M

There is to be 0.25 L of solution, so  

(0.0743 mole/L) (0.250 L) (60 g/mole) / (1.05 g/mL) = 1.054 mL.

The answer is b.