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6-2.Equilibrium-II (Ionic Equilibrium)
hard
What concentration of $Ac^-$ ions will reduce $H_3O^+$ ion to $2 × 10^{-4}\ M$ in $0.40\ M$ solution of $HAc$ ? $K_a (HAc) = 1.8 × 10^{-5}$ ?
A
$0.018\ M$
B
$0.00036\ M$
C
$00018\ M$
D
$0.036\ M$
Solution
$[A{c^ – }] = 0.036\,M$
$\mathop {HAc}\limits_{0.4(1 – \alpha )} \overset {Ka} \longleftrightarrow \mathop {{H^ + }}\limits_{0.4\,\alpha } + \mathop {A{c^ – }}\limits_{(b + 0.4\,\alpha )} $
$0.4 \alpha=2 \times 10^{-4}$
$\alpha=5 \times 10^{-4}$
$\mathrm{K}_{\mathrm{a}}=\frac{\mathrm{b} \times 0.4 \alpha}{0.4}=1.8 \times 10^{-5}$
$\mathrm{b}=\frac{1.8 \times 10^{-5}}{5 \times 10^{-4}}=0.036$
Standard 11
Chemistry
