Ncert Exercises & Solutions

Ionisation constant of a weak base MOH, is given expression

$K_{b} = \frac{[M^{+}] [{OH}^{-}]}{[MOH]}$

Values of ionisation constant of some weak bases at a particular temperature are given below

$\underset{K_{b}}{Base}$ $\underset{5.4 \times 10^{- 4}}{Dimethylamine}$ $\underset{1.3 \times 10^{- 14}}{Urea}$ $\underset{1.77 \times 10^{- 9}}{Pyridine}$ $\underset{1.77 \times 10^{- 5}}{Ammonia}$

Arrange the bases in decreasing order of the extent of their ionisation at equilibrium. Which of the above base is the strongest?

Given that, ionisation constant of a weak base MOH

K_{b} = \frac{[M^{+}] [{OH}^{-}]}{[MOH]}

Larger the ionisation constant (K_b) of a base, greater is its ionisation and stronger the base.

Hence, dimethyl amine is the strongest base.

Kb

\underset{5.4 \times 10^{- 4}}{Dimethylamine}

\underset{1.77 \times 10^{- 5}}{Ammonia}

\underset{1.77 \times 10^{- 9}}{Pyridine}

\underset{1.3 \times 10^{- 14}}{Urea}

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