MCQ
Ionisation constant of a weak acid $(HA)$ in terms of $\Lambda _m^\infty $ and ${\Lambda _m}$ is
  • A
    ${K_a} = \frac{{C\Lambda _m^\infty }}{{\left( {{\Lambda _m} - {\Lambda ^\infty }} \right)}}$
  • ${K_a} = \frac{{C\Lambda _m^2}}{{\Lambda _m^\infty \left( {\Lambda _m^\infty  - {\Lambda _m}} \right)}}$
  • C
    ${K_a} = \frac{{C{{\left( {\Lambda _m^\infty } \right)}^2}}}{{\Lambda _m^\infty \left( {{\Lambda _m} - {\Lambda ^\infty }} \right)}}$
  • D
    None of these

Answer

Correct option: B.
${K_a} = \frac{{C\Lambda _m^2}}{{\Lambda _m^\infty \left( {\Lambda _m^\infty  - {\Lambda _m}} \right)}}$
b
$\quad\quad\quad\quad\quad HA ( aq ) \rightleftharpoons H ^{+}( aq )+ A ^{-}( aq )$

At eqm. $\quad c (1-\alpha) \quad\quad\quad c \alpha \quad\quad\quad c \alpha$

$K _{ a }=\frac{ c \alpha^2}{1-\alpha} ;$ where $\alpha=\frac{\Lambda_{ m }}{\Lambda_{ m }^{\infty}}$

$\therefore \quad K _{ a }=\frac{ c \left(\frac{\Lambda_{ m }}{\Lambda_{ m }^{\infty}}\right)^2}{\left(1-\frac{\Lambda_{ m }}{\Lambda_{ m }^{\infty}}\right)}$

$=\frac{ c \Lambda_{ m }^2}{\Lambda_{ m }^{\infty}\left(\Lambda_{ m }^{\infty}-\Lambda_{ m }\right)}$

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