Question

One liter of a pH 5.00 propionic acid buffer with a total concentration of 50 mM must be prepared. Calculate the mmols propionic acid and sodium propionate needed to prepare the buffer. Ka= 1.34x10^-5 for propionic acid.

Answer 1

for a buffer...

acid +conjugate = 50*10^-3

for 1 Liter...

acid + conjguate = 50*10^-3 mol

the ratio:

pH = pKa + log(A-/HA)

pKa = -log(Ka) = -log(1.34*/10^-5) =4.87289

so

5 = 4.87289+ log(A-/HA)

A-/HA = 10^(5-4.87289) = 1.34

A = 1.34*HA

so..

acid +conjugate = 50*10^-3

HA + A- = 50*10^-3

substitute

A = 1.34*HA

HA + 1.34*HA= 50*10^-3

2.34HA =  50*10^-3

HA = ( 50*10^-3)/2.34 = 0.02136 mol = 0.02136*10^3 mmol = 21.36 mmol of propionic acid

A = 1.34*HA

[NaA] = 1.34*21.36 = 28.622 mmol of sodium propionate