## Calculating the Equilibrium Composition [ENDORSED]

Jennifer_Lam_1A
Posts: 21
Joined: Wed Sep 21, 2016 2:56 pm

### Calculating the Equilibrium Composition

How would you know if it's -x or +x for the change in molar concentration? (p. 137 of Course Reader)

Yiling Liu 1N
Posts: 25
Joined: Sat Jul 09, 2016 3:00 am

### Re: Calculating the Equilibrium Composition  [ENDORSED]

In the example you are referencing, the equation is PCl5 <--> PCl3 + Cl2

You are told that initially there is some amount of PCl5 in the reaction vessel. There is no other information in the problem that says the initial amount for PCl3 or Cl2. Thus, one assumes that PCl5 will decompose into the products in the forward reaction in order to reach equilibrium.

If PCl5 decomposes, then it will have some amount subtracted from the original amount of PCl5 there was. we can write that as "3-x", 3mol/L being the initial concentration.

Since there was some amount subtracted from it, that amount went in to the formation of the products, so both PCl3 and Cl2 had +x added to them.

Hope that helps!

Joseph Nguyen 3L
Posts: 24
Joined: Fri Jul 22, 2016 3:00 am

### Re: Calculating the Equilibrium Composition

It depends on the situation and what you are given. Obviously if you start with an intial concentration and they give you that of the reactant only and you have to assume the products are 0, then -x on products would give you a negative number, which isn't possible. Therefore, you know the reactant has to -x on reactants and -x on products. If they gave you initial concentrations of only the product, the opposite would be true.