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Structure of phosphoric acid is : (pK...

Structure of phosphoric acid is :
`(pK_(a1) = 2.12, pK_(2) = 7.21 , pK_(a3) = 12.32)`
First ionization of phosphoric acid is : `H_(3)PO_(4) hArr H_(3)PO_(4)^(-) + H^(+), pK_(a1) = 2.21`. The dissociation constant of conjugate base of `H_(2)PO_(4)^(-)` will be :

A

`6.45 xx 10^(-9)`

B

`1.62 xx 10^(-12)`

C

`3.48 xx 10^(-11)`

D

`4.62 xx 10^(-2)`

Text Solution

Verified by Experts

The correct Answer is:
B
`pK_(a_(1)) = 2.21, K_(a_(1))` = antilog `(-2.21) = 6.165 xx 10^(-3)`
`K_(b)` (of conjugate base) `= (K_(w))/(K_(a1)) = (110^(-14))/(6.165 xx 10^(-3)) = 1.62 xx 10^(-12)`
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The dissociation of weak electrolyte (weak acid) is expressed in terms of Ostwald dilution law. Stronger is the acid, weaker is its conjugate base. The dissociation constants of an acid (K_(a)) and its conjugate base (K_(b)) are related by the given relation : K_(w) = K_(a) xx K_(b) At 25^(@)C, K_(w) (Ionic product of water ) = 10^(14) . Phosphoric acid is a weak acid. it is used in fertilizer, food, detergent and toothpaste. Structure of phosphoric acid is : (pK_(a1) = 2.12, pK_(2) = 7.21 , pK_(a3) = 12.32) Aqueous solution of phosphoric acid with a density of 1 g mL^(-1) containing 0.05% by weight of phosphoric acid is used to impart taste to many soft drinks. Phosphate ion is an interfering radical in qualitative analysis. It should be removed for analysis beyond third group of qualitative analysis. Which among the following relations is correct ?

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