What is the mass?

Question

If you have 500  mL of a 0.10 M solution of the acid, what mass of the corresponding sodium salt of the conjugate base do you need to make the buffer with a pH of 3.50 (assuming no change in volume)?

pKa=3.34

Answer 1

Without knowing the acid you would not know the conjugate base. it would be possible to find the moles of the conjugate base but not the mass in grams as that would require converting the moles of the base found to mass knowing the molar mass of the base.

you can find the moles of the base from the following calculations using the Henderson Hasselbalch equation

PH = pKa + log Base /Acid

PH = 3.50

pKa = 3.34

weak acid 0.1M

3.5 = 3.34 + log Base/.1M

0.16 = log base/.1

antilog 0.16 = 1.445

1.445 = base/.1

0.1445M = Base

0.1445mol/L x 0.5 L = 0.72 moles of base

Again knowing it's a Na salt of the conjugate base, if you knew what the weak acid was you would have the exact formula for the Na salt of the conjugate base and since you know moles of base you could convert that to grams.