[JayEm]

Hi,
I was wondering if you could help me with the following question:

"When a mixture of 0.345 mol of PCl3 and 0.268 mol of Cl2 was heated in a vessel of fixed volume to a constant temperature, the following reaction reached equilibrium.

PCl3(g) + Cl2(g) <-> PCl5(g)

At equilibrium, 0.166 mol of PCl5 had been formed and the total pressure was 225 kPa.

Calculate the number of moles of PCl3 and of Cl2 in the equilibrium mixture."

The answers say that the no. of moles of PCl3 and Cl2 is 0.179mol and 0.102mol respectively. This is calculated by taking the no. of moles of PCl5 at EQ from the initial no. of mols of the the two reactants. But how can this be correct if you end up with less moles in the whole system than what you started with?

e.g. Initially you started with a total of 0.613mol of reactants, but at EQ, the total no. of moles in the system is 0.447mol.

 

[Borek]

What's the problem?

Synthesis of mercuric oxide:

2Hg + O₂ -> 2HgO

Decomposition of mercuric oxide:

2HgO -> 2Hg + O₂

In first case 3 moles give 2 moles, in second - 2 moles give 3.

Why do you suppose number of moles should not change? Mass, number of atoms - that's mass conservation law. But number of moles of reagents?

[Donaldson Tan]

mass is conserved in the system.

although the number of moles of reactants and products change, sum of mass of each component remains constant.

consider the reaction A + B -> C + D

the mass of C formed + the mass of D formed = mass of A reacted + mass of B reacted.