How many grams of solid sodium acetate should be added to 1.50L of a 0.287M acetic acid solution to prepare a buffer with pH 4.140? Grams sodium acetate=

Question

A. 2.43g

B. 8.77

C. 4.14

D. 8.00

Answer 1

One needs the pKa of acetic acid to answer this.  There are several values floating around, so I'm just using 4.74. Your mileage may vary if you use a different value.  Also, why give the pH to 4 significant figures when other values are 3 sig figs.  Tell your teacher this is an inane question.

pH = pKa + log [NaAc]/[HAc]

4.140 = 4.74 + log x

log x = -0.60

x = [NaAc]/[HAc] = 0.25

moles HAc = 1.50 L x 0.287 mol/L = 0.431 moles

moles NaAc = 0.25 x 0.431 = 0.1078 moles NaAc

grams NaAc = 0.108 x 80 g/mole = 8.64 grams (B?)