Question

A solution of acetic acid is prepared in water by adding 11.1 g of Na acetate to a volume flask and bringing the volume to 1.0 L with water. The final pH is measured to be 5.25. Assume temp 25C.

What are the concentrations of acetate and acetic acid in solution?

Please be a simple as possible. Thank you!

Answer 1

molar mass of sodium acetate = 82 g/mol
number of mol of sodium acetate = (given mass)/(molar mass)
= 11.1/82
= 0.14 mol
molarity of sodium acetate(M1) = (number of mole)/(volume of solution)
= 0.14/1
= 0.14 M

use,
pH = pKa + log([salt]/[acid])
pKa of acetic acid = 4.75
5.25 = 4.75 + log(0.14/[acid])
log(0.14/[acid]) = 0.5
(0.14/[acid]) = 10^0.5
(0.14/[acid]) = 3.16
[acid] = 0.14/3.16
= 0.044 M

Answer :
concentration of acetate = 0.14 M
concentration of acitic acid = 0.044 M