Question

you need to prepare a buffer with a pH of 12.20. start with 25.0 ml of 0.20 M HCl.

avaliable reagents : 1.00 M Na3PO4

1.00 M Na2HPO4

1.00 M NaCH3COO

a) choose weak acid/ conjugate base pair for this buffer

b) net ionic equation will be used to prepare this buffer

c)calculate volume of reagent that you must add to HCl soln. to prepare this buffer

Answer 1

a)

pKa3 of H3PO4 = 12.32

weak acid/conjugate base pair = 1.00 M Na3PO4 and 0.20 M HCl

b)

Net ionic equation :

PO43- (aq) +   H+ (aq)   <---------->   HPO42- (aq)

c)

PO43- (aq) +   H+ (aq)   <---------->   HPO42- (aq)

1.00 V              5.0                                 0

1.00V - 5           0                               5.0

mmoles of HCl = 25 x 0.20 = 5.0

mmoles of PO43- = 1.00 V

pH = pKa + log [salt / acid]

12.20 = 12.32 + log [PO43- / HPO42-]

12.20 = 12.32 + log [1.00 V - 5.0 / 5.00]

V = 8.79 mL

volume of reagent = 8.79 mL