General Chemistry 4th Edition University Science Books prese

General Chemistry 4th Edition University Science Books presented by Sapling Leaming Phosphorous acid, H3POs(aq), is a diprotic oxyacid that is pKa an important compound in industry and agriculture. pKaz 1.30 6.70 Calculate the pH for each of the following points in the titration of 50.0 mL of a 1.5 M H3PO3(aq) with 1.5 M KOH(aq). Number Phosphorous acid (a) before addition of any KOH Number (b) after addition of 25.0 mL of KOH D Number (c) after addition of 50.0 mL of KOHI Number (d) after addition of 75.0 mL of KOH I Number (e) after addition of 100.0 mL of KOH

Solution

H3PO3 millimoles = 50 x 1.5 = 75

a) Before any addition of KOH :

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

1.5                                       0             0

1.5- x                                   x              x

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

0.05 = x^2 / 1.5 - x

x = 0.25

[H+] = 0.25 M

pH = -log [H+] = -log [0.25]

       = 0.6

pH = 0.60

b) after addition of 25.0 mL KOH

it is first half equivaelce point

pH = pKa1 = 1.30

pH = 1.30

2 ) 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

3) 75.0 mL KOH

it is seond half equivalece point

pH = pKa2

pH = 6.70

4) 100.0 mL KOH

it is second equivalece point

HPO3^-2 millimoles = 100 x 1.5 =150

HPO3^-2 molarity = 150 / (50 +100) = 1.0 M

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

1.0 -x                                           x                x

Kb2 = x^2 / 1.4-x

5.01 x 10^-8 = x^2 / 1.0-x

x = 2.24 x 10^-4

[OH-] = 2.24x 10^-4 M

pOH = -log[OH-] = -log (2.24 x 10^-4 )

pOH = 3.65

pH + pOH = 14

pH = 10.35