Answer
$K=1.207$
Work Step by Step
The equilibrium constant is:
$K=[PCl_3][Cl_2]/[PCl_5]$
$K=1.3\times10^{-2}\ M\times3.9\times10^{-3}\ M/4.2\times10^{-5}\ M$
$K=1.207$
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