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At $250^oC$, phosphorus pentachloride is dissociated:
$PCl_5(g)$ 
$PCl_3(g)$ $+$ $Cl_2(g)$
A $5$ L container was filled with pure $PCl_5(g)$ and the heated to $250^oC$ until equilibrium was established.
The equilibrium molarities were:
$[PCl_5]$ $=$ $0.015\frac{mol}{L}$
$[PCl_3]$ $=$ $0.025\frac{mol}{L}$
$[Cl_2]$ $=$ $0.025\frac{mol}{L}$
Then the initial molarity of $PCl_5(g)$ must have been .......... $\frac{mol}{L}$
and the number of moles of $PCl_5(g)$ before dissociation was .......... mol.
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