Question

Phosphorous a and agriculture. The pK, values of phosphorous acid are cid, H, PO, (aq), is a diprotic oxyacid that is an important compound in industry HO-P-OH pKa 1.30 pKa2 6.70 Phosphorus acid Calculate the pH for each of the given points in the titration of 50.0 mL f 21 MH PO,(aq) with 2.1 M KOHag). pH before the addition of any KOH pH after the addition of 25.0 mL KOH pH after the addition of 50.0 mL KOH: pH after the addition of 75.0 mL KOH: pH after the addition of 100.0 mL KOH

Answer 1

1) Before any addition of KOH :

H3PO3 ---------------------> H+ +   H2PO3-

2.1                                       0             0

2.1- x                                   x              x

Ka1 = [H+][H2PO3-] / [H3PO3]

0.05 = x^2 / 2.1 - x

x = 0.30

[H+] = 0.30 M

pH = -log [H+] = -log [0.30]

       = 0.523

pH = 0.52

2) after addition of 25.0 mL KOH

it is first half equivaelce point

pH = pKa1 = 1.30

pH = 1.30

3 ) addition of 50.0 mL KOH

it is first equivalence point

pH = 1/2 (pKa1 + pKa2)

pH =1/2 (1.30 + 6.70)

pH = 4.0

4) 75.0 mL KOH

it is seond half equivalece point

pH = pKa2

pH = 6.70

5) 100.0 mL KOH

it is second equivalece point

HPO3^-2 millimoles = 100 x 2.1 = 210

HPO3^-2 molarity = 210 / (50 +100) = 1.4M

HPO3^-2 + H2O ------------------> H2PO4- + OH-

1.4 -x                                           x                x

Kb2 = x^2 / 1.4-x

5.01 x 10^-8 = x^2 / 1.4-x

x^2 + 5.01 x 10^-8 - 7.017 x 10^-8 = 0

x = 2.65 x 10^-4

[OH-] = 2.65 x 10^-4 M

pOH = -log[OH-] = -log (2.65 x 10^-4 )

pOH = 3.58

pH + pOH = 14

pH = 10.42