Question

Calculate the pH in 0.2000 M H3PO4. The stepwise dissociation constants are Ka1 = 7.50e-3 and...

Calculate the pH in 0.2000 M H3PO4. The stepwise dissociation constants are Ka1 = 7.50e-3 and Ka2 = 6.20e-8

2.125

1.454

1.045

2.103
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Answer #1

pH = ?

[H3PO4] = 0.2 M

Ka1= 7.50E-3

Ka2 = 6.2E-8

H3PO4 + H2O      H3O+ + H2PO4-

H3PO4 H2O H3O+ H2PO4-
Ci 0.2
Crx x
Cfinal 0.2-x x x

K1 = [H3O+ ][H2PO4-]/[H3PO4] = x2/0.2-x

K1(0.2-x) -x2 = 0 = 1.5E-3-7.5E-3x -x2= 0 ax2 + bx + c = 0

x = 0.035 = [H3O+]

-log [H3O+] = 1.45 = pH

H2PO4- + H2O ---------> H3O+ + HPO4-2

H2PO4- H2O H3O+   HPO4-2
Ci 0.035 0.035
Crx x
Cfinal 0.035-x 0.035+x x

K2 = [H3O+ ][HPO4-2]/[H2PO4-] = (0.035+x )(x)/0.035-x 0.035 -x = 0.035+x = 0.035

x=K2 = 6.2E-8 M

[H3O+ ] = 0.035+x = 0.035

-log [H3O+] = 1.45

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