Phosphorous acid, H3PO3(aq), is a diprotic oxyacid that is an important compound
ID: 907818 • 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.
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.
pH = pKa1 + pKa2 / 2
Explanation / Answer
C)2.4 M of H3PO3 = 2.4 M KOH
implies 2.4*98 g of phoporic acid = 2.4* 37 g of KOH
stoichiometrically 98 g of H3PO4 NEEDS 37g OF KOH
235.2 g of H3PO4 needs KOH = 88.8 g which is given and so it is exact equivalence point. here pH IS 7
d )After addition of 75 ml of KOH means wt of KOH = 25*2.4 M ( 75-50 , equivalcence pt)
so concentration of hydrocide ion excess = 25*2.4M OF KOH = (25/1000)*2.4 = 2.4/40 =0.06 M
pOH= -log 0.06 =2- 0.7781 = 1.2219
so pH = 12.78