Question

A buffer solution containing equal amounts of acetic acid and sodium acetate is prepared. What concentration of the buffer must be prepared to prevent a change in the pH by more than 0.20 pH units after the addition of 1 mL of 6M HClaq to 100mL of the buffer solution?

Answer 1

for a buffer

pH = pKa+ log(A-/HA)

find concentration of buffer in order to prevent

pH by 0.2 units...

if we added

mmol of HCl = MV = 6*1= 6

so..

for a volume of 100 mL

the chang ein pH will be due to:

H+ addition

so

mol of HA formed = 100*X + 6

mol of A- reacted = 100*X - 6

so

this change should not be greater than 0.20

so

pH = pKa + log(A/HA)

pH-pKa = log(A/HA)

0.2 =  log(A/HA)

10^-0.2 = (A/HA)

0.630 =  A/HA

[HA]*0.63 = [A-]

and we know

HA = 100*X + 6

A = 100*X - 6

so

[HA]*0.63 = [A-]

(100*X + 6 ) *0.63 = 100*X - 6

63*x + 6*0.63 = 100x - 6

(100-63)x =  6*0.63+6

x = 9.78/(100-63) = 0.26432

initial concentration of

[A-] = [HA] = X = 0.26432 M;