Question

A solution buffered at pH 3.90 is needed for a reaction. Would formic acid (HCOOH = HA) and its salt, sodium formate, NaHCO2 = NaA), make a good choice for this buffer? If so, what ratio of moles of A- and HA is needed? Use the Henderson Hasselbalch Equation. Ka formic acid = 1.8 x 10-4

Answer 1

The buffer is said to good buffer if the pka of the weak acid is closest to the required pH of the solution. Given that Ka for formic acid = 1.8 x 10-4 pka = -log(1.8 x 10-4) = 3.745 The pH of the solution =3.90 Since pH of the buffer solution required is close to the pka of formic acid. formic acid buffer (formic acid + sodium formate) can be used. We know that [base] pH = pka+log [acid] [HCOO ] > pH =- log (1.8x10*)+log [HCOOH] 33.90=3.745+log [HC00"] 10% [HCOOH] log HCOOH] [HCOO ] -0.155 [HCOO ] -100.155 = 1.429791 [HCOOH] ratio of A - to HA = 1.43