A liter of pH 7.20 phosphate buffer is needed for a certainexperiment. The Henderson-Hasselbach equation will be sufficientlyaccurate for your determination of pH. The pK’s for possiblyrelevant phosphate species are:
H3PO4 ↔ H2PO4- + H+ pK = 2.15
H2PO4- ↔ HPO4-2 + H+ pK = 7.20
HPO4-2 ↔ PO4-3 + H+ pK = 12.4
0.100 Moles of H3PO4 were dissolved in about 800 mL of water,and the pH was adjusted to 7.20 using a standardized pH meter and a1 M solution of KOH followed by addition of water to a volume of1.00 L.
a. (10 pts) Write the equation for the charge balance to showhow [K+], [H2PO4-], and [HPO4-2] are related. ([H+] and [OH-],although they are charged species, can be neglected for chargebalance when compared to [K+], [H2PO4-], and [HPO4-2])
b. (10 pts) At pH 7.20 what is the ratio of [H2PO4-] to[HPO4-2]?
c. (20 pts) Combine your findings from parts a and b todetermine how many mL of 1 M KOH must be added. {An important hint:By mass balance, the total molar concentration of phosphate speciesat pH 7.2 must equal the total molar concentration of phosphatespecies that was added from H3PO4.}
PLEASE ANSWER ALL PARTS, I NEED HELP. THANKS IN ADVANCE!
