Question

The first two questions pertain to the preparation of buffer using separate amounts of a weak acid and the salt of its conjugate base. Calculate the amounts of each needed to achieve the required concentrations or pH.

1) You need to prepare a 100 mL solution that is both 0.03 M sodium acetate and 0.02 M acetic acid. You have solid sodium acetate trihydrate and 1 M acetic acid solution. Show calculations and describe how to do this.

2) You need to make a solution that has pH of 4.6 from sodium acetate/acetic acid. Calculate the needed concentrations if the sum of the molarities of sodium acetate/acetic acid is 0.05 M.

Answer 1

1) vol = 100 ml = 0.1 L

moles of sodium acetate needed = M xV = 0.03 x 0.1 = 0.003

sodium acetate trihydrtate mass required = moles x moalr mass of it = 0.003 x 136.79 = 0.408 g

acetic acid moles = 0.02 x 0.1 = 0.002

Molarity = moles /vol

1 = 0.002 / vol

vol of acetic acid needed = 0.002L = 2 ml

thus 2 ml of 1 M acetic acid is taken and mixed with 0.408 g of sodium acetate trihydrtate and solution diluted to 100 ml to get buffer

2) pH = pka + log [sodium acetate] /[acetic acid]

4.6 = 4.745 + log [sodium acetate] /[acetic acid]

[sodium acetate] = 0.71614 [acetic acid] ....................(1)

[acetic acid ] + [sodium acetate] = 0.05 .............(2)

solving ( 1) and (2) we get

[acetic acid] = 0.029   ,

[sodium acetate] = 0.020865 M