Phosphoric acid is a triprotic acid with the following pK_a values: pK_a1 = 2.14
ID: 1050991 • Letter: P
Question
Phosphoric acid is a triprotic acid with the following pK_a values: pK_a1 = 2.148 pK_a2 = 7.198 pK_a3 You wish to prepare 1.000 L of a 0.0400 M phosphate buffer at pH 7.640. To do this, you choose to mix the two salt forms involved in the second ionization, Na_2H_2PO_4 and Na_2HPO_4, in a 1.000 L volumetric flask and add water to the mark. What mass of each salt will you add to the mixture? What other combination of phosphoric acid and/or its salts could be mixed to prepare this buffer? (Check all that apply).Explanation / Answer
____ A. H3PO4 and NaH2PO4
__X__ B. H3PO4 and Na2HPO4------------answer
__X__ C. H3PO4 and Na3PO4-------answer
___X_D. NaH2PO4 and Na3PO4-------answer
____E. Na2HPO4 and Na3PO4
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Q1
0.755g of NaH2PO4
and
0.527g of Na2HPO4---answer
pKa1 = 2.148 = [H+][H2PO4 -] / [H3PO4]
2.148 = x^2 / 0.01 - x
x^2 + 2.148x - 0.02148 = 0
x = [H2PO4-] = 0.00995M
pH = pKa + log[Na2HPO4]/[NaH2PO4]
by plugging value sin the above equation
0.755g of NaH2PO4
and
0.527g of Na2HPO4---answer
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