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The molar conductivities of H^(+) and HC...

The molar conductivities of `H^(+) and HCOO^(-)` ions at infinite dilution are 34.7 and 5.4 `mSm^(2) " mol"^(-1)` respectively. The molar conductivity of 0.25 M HCOOH is `4.0 mSm^(2) mol^(-1)`. Then `pK_(a)` of formic acid is

A

3.6

B

4.2

C

4.8

D

1.8

Text Solution

Verified by Experts

The correct Answer is:
A
`alpha = (^^_M)/(^^_(M)^(0)) = (4)/(34.6 + 5.4) = (4)/(210) = 0.1 , K_a = C alpha^(2) = 0.025 xx (0.1)^(2) = 25 xx 10^(-5)`
`P^(Ka) = - log(25 xx 10^(-5)) = 5 - log 5^(2) = 5 = 2 log 5 = 5 -2 xx 0.7 = 5 - 1.4 = 3.6`
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