JEE Main 2025 Jan 23 Shift 1, Chemistry Q21: Properties of Amines

JEE Main2025Jan 23, Shift 1Chemistry

If 1 mM solution of ethylamine produces pH = 9, then the ionization constant ( $K_{b}$ ) of ethylamine is $1 0^{- x}$ . The value of x is ________ (nearest integer).

[The degree of ionization of ethylamine can be neglected with respect to unity.]

Your answer

Solution

The equilibrium is $C_{2} H_{5} N H_{2} + H_{2} O ⇌ C_{2} H_{5} N H_{3}^{+} + O H^{-}$ with initial concentration $C = 1 0^{- 3}$ M.

Given pH $= 9$ , so pOH $= 5$ and $[O H^{-}] = 1 0^{- 5}$ M.

Since $α ≪ 1$ , $[C_{2} H_{5} N H_{3}^{+}] = [O H^{-}] = 1 0^{- 5}$ M and $[C_{2} H_{5} N H_{2}] \approx 1 0^{- 3}$ M.

$K_{b} = \frac{[ C _{2} H _{5} N H _{3}^{+} ] [ O H ^{-} ]}{[ C _{2} H _{5} N H _{2} ]} = \frac{( 1 0 ^{- 5} ) ( 1 0 ^{- 5} )}{1 0 ^{- 3}} = 1 0^{- 7}$ .

$∴ x = 7$ .

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JEE Main 2025 Jan 23 Shift 1, Chemistry Q21: Properties of Amines

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