Question

$BaC{l}_2$ dissociates in water to give one $B{a}^{2+}$ion and two $C{l}^{-}$ ions. If concentrated $HCl$ is added to this solution:

• A. $\left[B{a}^{2+}\right]$ increases
• B. $\left[B{a}^{2+}\right]$ remains constant
• C. $\left[O{H}^{-}\right]$ increases
• D. the number of moles of undissociated $BaC{l}_2$ increases

Answer 1

The correct option is D the number of moles of undissociated $BaC{l}_2$ increases

$BaC{l}_2$ dissociates to give one $B{a}^{2+}$ and two $C{l}^{-}$ ions in water.

This can be written as $BaC{l}_2$$\leftrightharpoons$$B{a}^{2+}$$+$$2C{l}^{-}$

Now, when we add $HCl$ in this solution, concentrated $HCl$ will dissociate as $H^{+}$ and $C{l}^{-}$, which in turn will increase the concentration of $C{l}^{-}$ ions in the solution.

But according to Le Chatelier's principle, when a system at equilibrium is subject to any change in concentration, temperature, pressure or volume, the system readjusts itself to counteract the effect of the applied change and a new equilibrium is established.

So, since the concentration of $C{l}^{-}$ ions is increased now, the system will try to decrease the concentration of $C{l}^{-}$ ions, and hence the number of moles of undissociated $BaC{l}_2$ ions will increase.

Option D is the correct answer.