Phosphorous acid, H3PO3(aq), is a diprotic oxyacid that is an important compound
ID: 907864 • Letter: P
Question
Phosphorous acid, H3PO3(aq), is a diprotic oxyacid that is an important compound in industry and agriculture. Calculate the pH for each of the following points in the titration of 50.0 mL of a 2.4 M H3PO3(aq) with 2.4 M KOH(aq).
a) before addition of any KOH = 0.5
b) after addition of 25.0 mL of KOH = 1.3
c) after addition of 50.0 mL of KOH =
d) after addition of 75.0 mL of KOH =
e) after addition of 100.0 mL of KOH = 10.2
answers to a,b,and e are correct can't figure out c and d.
The answer to c is not 7 and the answer to d is not 12.78
hint: Point (c) is the first equivalence point (because we've added equal volumes of equimolar acid and base). The product of the reaction is H2PO3–, which is amphoteric. The pH of an amphoteric substance can be calculated using the following formula.
Explanation / Answer
c) after addition of 50.0 mL of KOH
it is the first equivaelce point at this point pH = (pKa 1 + pKa2 ) / 2
= (1.30 + 6.70 ) /2
= 4
d) after addition of 75.0 mL of KOH
75 ml KOH added mean . it is the second half - equivalence point
at this point pH =pKa2
pH = 6.70