Question

Phosphorus pentachloride gas decomposes to form phosphorus trichloride gas and chlorine gas. What is the equilibrium constant for this reaction if a reaction vessel has an initial concentration of phosphorus pentachloride of 0.500 M and the equilibrium concentration of 0.119 M? Neither chlorine gas nor phosphorus trichloride are present in the initial reaction vessel. Assume a 1.00 L reaction vessel.

Thank you

Answer 1

The given chemical reaction in equilibrium is

PCl5(g) -------> PCl3(g) + Cl2(g), Kc = ?

In the above reaction, 1 mol of PCl5 decompose to give 1 mol of PCl3 and 1 mol of Cl2.

PCl5(g) ------> PCl3(g) + Cl2(g), Kc = ?

Initial moles : 0.500Mx1.00 0 0

= 0.500 mol 0 0

mol at eqm: (0.500 - x)mol x mol x mol

Given equilibrium moles of PCl5 = (0.500 - x)mol = 0.119 Mx1.00L = 0.119 mol

=> x = (0.500 - 0.119) mol = 0.381 mol

Hence equilibrium moles of PCl3 = x = 0.381 mol

equilibrium moles of Cl2 = x = 0.381 mol

Equilibrium concentration of PCl3, [PCl3] = (moles of PCl3) / Volume = 0.381 mol / 1.0 L = 0.381M

Equilibrium concentration of Cl2, [Cl2] = (moles of Cl2) / Volume = 0.381 mol / 1.0 L = 0.381M

Now Kc can be calculated as

Kc = [PCl3]*[Cl2] / [PCl5] = 0.381M*0.381M / (0.119M) = 1.22 (answer)