phosphoric acid is a diprotic acid, and a 0.20 m, 50.0ml sample is titrated with

Question

phosphoric acid is a diprotic acid, and a 0.20 m, 50.0ml sample is titrated with 12.5 ml of 0.100m potassium hydroxide to reach the first equivalent point, where the first acid dissociation constant ka1= 7.25x10^-3m at 25c. an additional 12.5 ml, 0.100 mof potassium hydroxide is added to reach the first equivalence point, where the second acid dissociation constant is ka2= 6.31x10^-8m at 25c, and the acid is neutralized.

a) write a balance chemical equation including all phasesb) calculate the pH of the first equivalent point

b) Calculate the PH for the first equivalent point

c) write the balance chemical equation for the second dissociation

Explanation / Answer

(a) H3PO4 (aq) + KOH (aq) ---------> KH2PO4 (aq) + H2O (aq)

(b) pH at first equivalnce point = [(pka1 + pka2)/2] (only for diprotic acid)

Given Ka = ka1= 7.25x10^-3

so pKa1 = 2.14

Also, ka2= 6.31x10^-8

pKa2 = 7.2

pH at first equivalence = avearge of two pKa

pH = (2.14 + 7.2)/2 = 4.67

(c) H3PO4 (aq) + 2KOH (aq) ---------> K2HPO4 (aq) + 2H2O (aq)

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