Question

Phosphorus pentachloride decomposes at higher temperatures.

PCl5(g) ⇄ PCl3(g) + Cl2(g)

An equilibrium mixture at some temperature consists of

5.93 g PCl5, 208.23 g/mol

4.86 g PCl3, 137.33 g/mol

3.59 g Cl2, 70.91 g/mol

in a 1.00-L flask.

If you add 1.31 g of Cl2, how will the equilibrium be affected and what will the concentration of PCl5 be when equilibrium is reestablished?

shift left

shift right

no shift will occur

[PCl5] = mol/L

Answer 1

At first equilibrium, the concentrations are calculated as shown below

The equilibrium constant

1.31 g of chlorine is added. New concentration of chlorine is

The equilibrium will shift to left. More and more chlorine will combine with phosphorus trichloride to form more and more phosporous penta chloride.

Let x M of chlorine dissociates to reach second equilibrium. Prepare an ICE table

Initial concentration (M)	0.02848	0.03539	0.0691
Change in concentration (M)	+x	-x	-x
Equilibrium concentration (M)

Substitute values in equilibrium constant expression

This is quadratic equation with solution

The negative value is discarded as concentration cannot be negative

At second equilibrium,