Question

In many separations pH control is necessary. Assume you want to make a buffer at pH=4 using glacial acetic acid and 10 M NaOH. You will need two liters at 0.1 M total buffer concentration. How much of each component will you use (Ka=1.8*10^-5)

Answer 1

ph = 4

V = 2L, 0.1 M total

total mol = MV = 2*0.1 = 0.2 mol required

A- + HA = 0.2

from pH, get A-/HA ratio

pH = pKa + log(a-/HA)

4 = 4.75 + log(A-/HA)

A-/Ha = 10^(4-4.75) = 0.1778

now..

from

A- + HA = 0.2

HA = 0.2 - A

A-/HA = 0.1778

A- = (0.1778) *  0.2 - A

A-/0.1778 = 0.2 -A

A-*(1+1/0.1778) = 0.2

A- = 0.2/(1+1/0.1778) = 0.03019 M

now...

HA = 0.2 - A = 0.2 - 0.03019

HA = 0.16981

Total mol of HA + A- = 0.2

assume initially, we have glacial acetic acid

mass of glacial acetic acid = mol*MW = 0.2*60 = 12 g of glacial acetic acid initially

the, add

0.03019 mol ofNaOH

V = mol/M = 0.03019 /10 = 0.003019 L = 0.003019*!0^3 mL = 3.02 mL of NAOH

mix in V = 2Liters