Suppose \(K=4.5 \times 10^{-3}\) at a certain temperature for the reaction $$\mathrm{PCl}_{5}(g) \rightleftharpoons \mathrm{PCl}_{3}(g)+\mathrm{Cl}_{2}(g)$$ If it is found that the concentration of \(\mathrm{PCl}_{5}\) is twice the concentration of \(\mathrm{PCl}_{3},\) what must be the concentration of \(\mathrm{Cl}_{2}\) under these conditions?

Short Answer

Expert verified
The concentration of $\mathrm{Cl}_{2}$ under these conditions is: $$[\mathrm{Cl}_{2}] = 9 \times 10^{-3} \mathrm{M}$$

Step by step solution

01

Write the equilibrium constant expression

For the given reaction: $$\mathrm{PCl}_{5}(g) \rightleftharpoons \mathrm{PCl}_{3}(g) + \mathrm{Cl}_{2}(g)$$ We can write the equilibrium constant expression (K) as: $$K = \frac{[\mathrm{PCl}_{3}][\mathrm{Cl}_{2}]}{[\mathrm{PCl}_{5}]}$$
02

Use the given information to create a relationship between the concentrations

We are informed that the concentration of PCl5 is twice the concentration of PCl3, which can be represented as: $$[\mathrm{PCl}_{5}] = 2\cdot [\mathrm{PCl}_{3}]$$
03

Replace the relationship in the equilibrium constant expression

Substitute the relationship between PCl5 and PCl3 concentrations in the equilibrium constant expression: $$K = \frac{[\mathrm{PCl}_{3}][\mathrm{Cl}_{2}]}{2\cdot[\mathrm{PCl}_{3}]}$$
04

Simplify and solve for the concentration of Cl2

Simplify the expression and solve for the concentration of Cl2: $$4.5 \times 10^{-3} = \frac{[\mathrm{Cl}_{2}]}{2}$$ Multiply both sides by 2 to isolate the concentration of Cl2: $$[\mathrm{Cl}_{2}] = 2 \times 4.5 \times 10^{-3}$$ Calculate the concentration of Cl2: $$[\mathrm{Cl}_{2}] = 9 \times 10^{-3} \mathrm{M}$$
05

Write the final answer

The concentration of Cl2 under these conditions is: $$[\mathrm{Cl}_{2}] = 9 \times 10^{-3} \mathrm{M}$$

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