Question

you need to prepare a buffer solution with a ph of 7.8. A lab mate tells you to use N-2- hydroxyethylpiperazine-N'-2-ethanesulfonic acid(HEPES), a sulfonic acid with a pka of 7.5. since you know that the buffer pka should be close to the desired pH, this sounds good. You make the solution according to the following recipe, which is supposed to make 600 ml of a 0.5 M solution:

dissolve 0.2 moles of HEPES in 400 ml of water. add 0.1 moles of the na-HEPES( the sodium salt of the conjugate base) and dissolve. add water to a total volume of 600 ml.

a) before adding water, you check the pH, and it looks like your labe mate gave you the incorrect recipe. calculate the expected pH for the solution made according to the recipe.

b)now you want to change the pH to the desired 7.8 but you don't want to add any more HEPES or na-HEPEs. you have stock solutions of 1M HCL and 1M NaOH. which of these solutions should you add to change the pH to 7.8? eplain your reasoning.

c)how many ml of the 1M HCL or NaOH solution should you add?

Answer 1

a) pH = pKa + log([base]/acid])

[HEPES] = 0.2 mol/0.6 L = 0.333 M

[NaHEPES] = 0.1 mol/0.6 L = 0.166 M

pH of buffer thus prepared would be,

pH = 7.5 + log(0.166/0.333) = 7.20

b) To change the pH to 7.8, we must add base 1 M NaOH to the buffer. The required pH is higher than the present pH and we need more conjugate base to form. Thus we would have to add NaOH to the solution.

c) we have,

[HEPES] + [NaHEPES] = 0.5 M

and,

7.8 = 7.5 + log([NaHEPES]/[HEPES])

0.3[HEPES] = [NaHEPES]

[HEPES] + 0.3[HEPES] = 0.5

[HEPES] = 0.385 M

[NaHEPES] = 0.115 M

Volume of NaOH required = 0.015/1 = 0.015 L = 15.0 ml of 1 M NaOH