You are instructed to create 100. mL of a 0.47 M phosphate buffer with a pH of 7
ID: 898032 • Letter: Y
Question
You are instructed to create 100. mL of a 0.47 M phosphate buffer with a pH of 7.6. You have phosphoric acid and the sodium salts NaH2PO4, Na2HPO4, and Na3PO4 available. (Enter all numerical answers to three significant figures.) H3PO4(s) + H2O(l) equilibrium reaction arrow H3O+(aq) + H2PO4(aq) Ka1 = 6.9103 H2PO4(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + HPO42(aq) Ka2 = 6.2108 HPO42(aq) + H2O(l) equilibrium reaction arrow H3O+(aq) + PO43(aq) Ka3 = 4.81013
What is the molarity needed for the acid component of the buffer?
What is the molarity needed for the base component of the buffer?
How many moles of acid/base are needed for the buffer?
How many grams of acid/base are needed for the buffer?
Explanation / Answer
1) for desired pH of 7.6 , we have to take acid pKa2 = -log (6.2 108 ) = 7.2
for a phospate buffer pH =pKa2 + log(Salt/Acid)
7.6 = 7.2 + log(Salt/Acid)
log( salt/acid) = .4
taking antilog [salt]/[acid] = 2.51
its the ratio of salt : acid that should be present in the solution for desired 7.6 pH.
given molarity for phosphate [phosphate] salt = .47 M base component
so [acid] = .47/2.51 = .187 acid component
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