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MusicMonkey
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According to theory, which amounts of the following acid and its conjugate base would you need to prepare 150 ml of a 0.3 M phosphate buffer at pH 7.9? Use the pKa table below to help you in your answer.
H3PO4 <–> H2PO4- + H+ pKa = 2.3
H2PO4- <–> HPO4-2 + H+ pKa = 7.2
HPO4-2 <–> PO4-3 + H+ pKa = 12.1
Molecular weights: PO4-3 = 95, Na = 23, H = 1
a) 7.5 ml of 0.3M NaH2PO4 and 37.5 ml of 0.3M Na2HPO4 plus water
b) 37.5 ml of 0.3M NaH2PO4 and 7.5 ml of 0.3M Na2HPO4 plus water
answer:--> c) 25 ml of 0.3M NaH2PO4 and 125 ml of 0.3M Na2HPO4
d) 125 ml of 0.3M NaH2PO4 and 25 ml of 0.3M Na2HPO4
e) 0.05 ml of 0.3M NaH2PO4 and 0.25 ml of 0.3M Na2HPO4 plus water
f) 0.25 ml of 0.3M NaH2PO4 and 0.05 ml of 0.3M Na2HPO4 plus water
I don't understand why c) would be the correct answer. Please help!
H3PO4 <–> H2PO4- + H+ pKa = 2.3
H2PO4- <–> HPO4-2 + H+ pKa = 7.2
HPO4-2 <–> PO4-3 + H+ pKa = 12.1
Molecular weights: PO4-3 = 95, Na = 23, H = 1
a) 7.5 ml of 0.3M NaH2PO4 and 37.5 ml of 0.3M Na2HPO4 plus water
b) 37.5 ml of 0.3M NaH2PO4 and 7.5 ml of 0.3M Na2HPO4 plus water
answer:--> c) 25 ml of 0.3M NaH2PO4 and 125 ml of 0.3M Na2HPO4
d) 125 ml of 0.3M NaH2PO4 and 25 ml of 0.3M Na2HPO4
e) 0.05 ml of 0.3M NaH2PO4 and 0.25 ml of 0.3M Na2HPO4 plus water
f) 0.25 ml of 0.3M NaH2PO4 and 0.05 ml of 0.3M Na2HPO4 plus water
I don't understand why c) would be the correct answer. Please help!