Question

The dissociation constant of a weak acid is $1.0\times {10}^{-5}$. The equilibrium

constant for the reaction with strong base is

• A. 1.0 x 10^-5
• B. 1.0 x 10^-9
• C. 1.0 x 10⁹
• D. 1.0 x 10¹⁴

Answer 1

The correct option is C

1.0 x 10⁹

$HA\leftrightharpoons H^{+}+A^{-};K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[HA\right]}$ ….(i)
neutralization of the weak acid with strong base is
$HA+O{H}^{-}\leftrightharpoons A^{-}+H_{2}O$ ….(ii)
$K=\frac{\left[A^{-}\right]}{\left[HA\right]\left[O{H}^{-}\right]}$
dividing (i) by (ii) $\frac{K_a}{K}=\left[H^{+}\right]\left[O{H}^{-}\right]=K_w={10}^{-14}$
$K=\frac{K_a}{K_w}=\frac{{10}^{-5}}{{10}^{-14}}={10}^9$