Question

Phosphorus pentachloride decomposes according to the chemical equation

PCl5 <-----> PCl3 + Cl2

Kc=1.80 at 250 °C

A 0.135 mol sample of PCl5(g) is injected into an empty 2.15 L reaction vessel held at 250 °C. Calculate the concentrations of PCl5(g) and PCl3(g) at equilibrium.

Answer 1

Solution :-

Balanced reaction equation

PCl5 <-----> PCl3 + Cl2

Kc= 1.80

Lets calculate the initial concentration of the PCl5 at the initial

Molarity = moles / volume in liter

[PCl5]= 0.135 mol / 2.15 L

          = 0.0628 M

Now lets set up the ICE table

PCl5     <----->     PCl3    +   Cl2

0.0628 M               0             0

-x                            +x            +x

0.0628-x               x              x

Kc= [PCl3][Cl2]/[PCl5]

1.80= [x][x]/[0.0628-x]

1.80*[0.0628-x] =x^2

Solving for the x we get

0.0607 M=x

So the equilibrium concentrations of the PCl3 and Cl2 are as follows

[PCl3]eq = x = 0.0607 M

[Cl2]eq = x = 0.0607 M