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Question

Calculate the pH of a 0.05 M weak monobasic acid solution. The value of dissociation constant is 8.0×109.

A
2
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B
4.69
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C
6.6
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D
6.37
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Solution

The correct option is B 4.69
Given,
Ka=8.0×109, concentration (C) = 0.05 M
Let the weak monobasic acid is HA . The dissociation of weak acid is given by :
HA H++ A C 0 0CCα Cα Cα

Ka=Cα2C(1α)As,α<<1Ka=Cα2
α=Kacα=8×1090.05=16×108=4×104
Concentration of [H+]=Cα=(0.05×4×104)=2×105 mol L1
pH=log[2×105]=4.69

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