First slide

Ionization constants of weak acids, PH of weak acids

Question

The correct expression for OStwald's dilution law for a solution of weak electrolyte is (if initial number of moles = 1)

Easy

A

$\large {K_a}\, = \,\frac{{{\alpha ^2}}}{{\left( {1 - \alpha } \right)V}}$
B

${K_a}\, = \,{\alpha ^2}.V$
C

${K_a}\, = \,\frac{{{\alpha ^2}}}{{1 - V}}$
D

${K_a}\, = \,\frac{{{\alpha ^2}}}{{C(1 - \alpha )}}$

Solution

      ${CH}_{3} COOH ⇌ {CH}_{3} {COOO}^{-} + H^{+}$
t=0 C₀ 0 0
t=t_eq C₀-C₀α C₀α C₀α
${K_a} = \frac{{\left ({C_0}\alpha \right ) \left ({C_0}\alpha \right )}}{{{C_0} - {C_0}\alpha }}$
$= \frac{{{C_0}{\alpha ^2}}}{{1 - \alpha }}$

$\rightarrow$
(1)
1 - α ~ 1
$\alpha = \sqrt {\frac{{{K_a}}}{{{C_0}}}} = \sqrt {\frac{{{K_a}V}}{1}}$
$\left( 1 \right) \Rightarrow {K_a} = \frac{{{\alpha ^2} \times 1}}{{\left (1 - \alpha \right )V}}$

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