PCl5, PCl3, and Cl2 are at equilibrium at 500 K in a closed container and their concentrations are 0.8×10–3 mol L–1 , 1.2×10–3 mol L–1 and 1.2×10–3 mol L–1, respectively.
The value of  Kc  for the reaction PCl5(g) ⇌ PCl3(g) + Cl2(g) will be:
1. 1.8 × 103 mol L–1
2. 1.8 × 103
3. 1.8 × 10–3 mol L–1
4. 0.55 × 104

Hint: The formula of Kc=[PCl3][Cl2][PCl5]
Step 1:
For the reaction, PCl5(g)PCl3(g)+Cl2(g)
The given values are as follows:
At 500 K in a closed container, [PCl5]=0.8×10-3 mol L-1
[PCl3]=1.2×10-3 mol L-1
[Cl2]=1.2×10-3 mol L-1
Step 2:
\(\begin{aligned} & \mathrm{K}_{\mathrm{c}}=\frac{\left[\mathrm{PCl}_3\right]\left[\mathrm{Cl}_2\right]}{\left[\mathrm{PCl}_5\right]} \\ & =\frac{\left(1.2 \times 10^{-3}\right) \times\left(1.2 \times 10^{-3}\right)}{\left(0.8 \times 10^{-3}\right)} \\ & =1.8 \times 10^{-3}~ \text {mol L}^{-1} \end{aligned}\)