In: Chemistry
. On your first day working in a biochemistry research lab, you are asked to prepare 500 mL of a 0.15 M sodium phosphate buffer solution at pH 6.5. Your lab contains stock solutions of H3PO4, NaH2PO4, Na2HPO4, and Na3PO4, each at 0.5 M, as well as plenty of double-distilled water.(a). (6 points) What do you mix and in what amounts in order to prepare the desired buffer?
total no of mol of phosphatebuffer = M*V = 0.15*0.5 = 0.075 mol
pH of phosphate buffer = pka2 + log[Na2HPO4/NaH2PO4]
pka2 of H3PO4 = 7.2
[Na2HPO4] = x
[NaH2PO4] = 0.032-x
6.5 = 7.2 + log(x/(0.075-x))
x = 0.0125
[Na2HPO4] = x = 0.0125 mol
[NaH2PO4] = 0.075-x = 0.075-0.0125 = 0.0625 mol
volume of Na2HPO4 stock solution must take = n/M = 0.0125/0.5 = 0.025 L
= 25 ml
volume of NaH2PO4 stock solution must take = 0.0625/0.5 = 0.125 L
= 125 ml
25 ml +125 ml + rest add water up to 500 ml
so that desired buffer will be prepared.