Find Equilibrium Constant with only balanced equation?


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> Calculate the value of $K\_p$ for the following equation:
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> $$\ce{PCl3 (g) + Cl2 (g) <=> PCl5 (g)} ,\qquad \mathrm{R = 0.0821 \frac{L\times atm}{mol\times K}}$$
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This question appeared on my test. I know how to find $K\_p$ if I was given $K\_c$ $(K\_{\mathrm{p}} = K\_\mathrm{c} \times (\mathrm{R} \times T)^{n})$ or if I was given the partial pressures of all of the parts at equilibrium, but none of those values were given; only the equation. My professor marked the answer as $0.041$ (no units), but I have no idea how to calculate that if I wasn't given any values.


