Question

BIOCHEM:

Acetic acid has a pKa of 4.76. Starting with a 1.0 M solution of sodium acetate, and a 1.0 M solution of acetic acid, what volume of each is required to make 1.0 liter of a 1.0 M solution buffered at pH 5.1?

Will give a thumbs up if correct. I am not sure what to put in for "([A-/HA])" from the Henderson-Hasselbalch equation. Please do explain, thank you.

Answer 1

let acetic acid be represented as HA and its ionization in water is HA+ H2O------>A- ( conjugate base)+ H3O+

Henderson-Hasselbalch equation can be written as

pH= pKa+ log [A-]/[HA]

5.1= 4.76+ log [A-]/[HA], [A-]/[HA]= 2.2

let x= volume of acetic acid , 1-x= volume of sodium acetate

moles of acetic acid in 1 L of 1M= x and moles of sodium acetate in 1L of 1M= (1-x)*1

concentrations : [HA] =x and [A-]= (1-x)/1

hence (1-x)/x= 2.2

1-x= 2.2x, 3.2x= 1, x= 1/3.2= 0.3125,   1-x= 1-0.3125=0.6875

volume of acetic acid = 0.3125L and volume of sodium acetate= 0.6875L