Question

In an aqueous solution of volume $500\, mL ,$ when the reaction of $2 Ag ^{+}+ Cu \rightleftharpoons Cu ^{2+}+2 Ag ,$ reached equilibrium the $\left[ Cu ^{2+}\right]$ was $x\, M$. When $500\, mL$ of water is further added, at the equilibrium $\left[ Cu ^{2+}\right]$ will be

• A. 2 x M
• B. x M
• C. Between x M and x / 2 M
• D. Less than x / 2 M

Answer 1

Answer: (D)

$\underset{aM}{2 Ag ^{+}}+ Cu \rightleftharpoons \underset{xM}{Cu ^{2+}}+ \underset{(V=500\, mL )}{2 Ag}$

$K_{c}=\frac{\left[ Cu ^{2+}\right]}{\left[ Ag ^{+}\right]^{2}}=\frac{x}{a^{2}}$

Also $K_{c}=\frac{\left[ Cu ^{2+}\right]}{\left[ Ag ^{+}\right]^{2}}=\frac{\left[ Cu ^{2+}\right]}{\left(\frac{a}{2}\right)^{2}}$ when $500\, mL$ is further added.

Thus to have $K_{c}$ constant $\left[ Cu ^{2+}\right]$ must be less than $x / 2$.