Equilibrium constant for the thermal dissociation of phosphorus pentachloride
Note the equation as follows: $$\ce{PCl5 &lt;=&gt; Cl2 + PCl3}$$

You may begin with an ICE table showing the initial and final pressures of the gases.
$$\begin{array}{|c|c|c|} \hline
 &amp; \ce{PCl5}  &amp; \ce{PCl3} &amp;\ce{Cl2} \\ \hline
\text{Initial} &amp; 2 &amp; 0&amp;0 \\ \hline
\text{Change} &amp; -1 &amp; +1 &amp; +1 \\\hline
\text{Equilibrium} &amp; 1 &amp; 1 &amp;1 \\ \hline
\end{array}$$
Then it is a simple matter to find $K_p$ with the equation $$K_p = \frac{[\ce{P_{PCl_3}}][\ce{P_{Cl_2}}]}{[\ce{P_{PCl_5}}]}$$

If you would like $K_c$ you would need the ideal gas law ($PV=nRT$) but I doubt that is what your question requires.