1. Phosphoric acid Write the 3 equilibrium reactions for the dissociation of pho
ID: 1083032 • Letter: 1
Question
1. Phosphoric acid Write the 3 equilibrium reactions for the dissociation of phosphoric acid in water. Use the CRC or the textbook to write the corresponding pKa values next to each reaction. Identify the strongest acid. a. Calculate the pkb values and write the 3 equilibrium reactions that correspond with the calculated pKb values. Identify the strongest base. b. When KH,PO, is placed in water, would you expect the resulting solution to be acidic or basic? Why? c. When K,HPO, is placed in water, would you expect the resulting solution to be acidic or basic? Why? d. a + Page 7 1 7Explanation / Answer
Phosphoric acid (H3PO4)
a. Dissociation of weak acid (phosphoric acid)
Ist dissociation,
H3PO4 <==> H2PO4- + H+ pKa 2.15
IInd dissociation,
H2PO4- <==> HPO4^2- + H+ pKa 7.20
IIIrd dissociation,
HPO4^2- <==> PO4^3- + H+ pKa 12.35
The strongest acid among all of them is H3PO4, which dissociates to a great extent with a pKa of 2.15 (lowest pKa).
b. the pKb can be obtained from the pKa values,
Ist base equation,
PO4^3- + H+ <==> HPO4^2- pKb = 14 - 12.35 = 1.65
IInd base equation,
HPO4^2- + H+ <==> H2PO4^- pKb = 14 - 7.20 = 6.80
IIIrd base equation,
H2PO4^- + H+ <==> H3PO4 pKb = 14 - 2.15 = 11.85
The strongest base among these would have lowest pKb that is PO4^3- with an pKb of 1.65
c. When KH2PO4 is placed in water, the resulting solution would be acidic. The pKa of H2PO4- is lower than the pKb, that is it is a stronger acid than acting as base.
therefore the equilibrium reaction in solution would be,
H2PO4- + H2O <==> HPO4^2- + H3O+ (acid)
d. When K2HPO4 is placed in water, the resulting solution would be basic as pKb is lower than the pKa of HPO4^2- species.
therefore the equilibrium reaction in the solution would be,
HPO4^2- + H2O <==> H2PO4- + OH- (base)
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