# Homework Help: Applications of Equilibrium Constants

1. Jan 16, 2010

### erok81

1. The problem statement, all variables and given/known data

At 1285*C the equilibrium constant for the reaction $$Br_2 (g) \Leftrightarrow 2Br (g)$$. A 0.200L vessel containing an equilibrium mixture of the gasses has 0.245g of $$Br_2$$(g) in it. What is the mass of Br(g) in the vessel?

2. Relevant equations

None

3. The attempt at a solution

I have tried this 4-5 different ways at least and cannot get close to the answer. I started with this

$$K_c = \frac{[Br]^{2}}{[Br_2]}$$

Rearrange that to solve for Br. I get...

$$[Br_2] \times K_c = [Br]^2$$

Then after that I am lost. I've tried converting Br_2 from grams to moles. Leaving it as grams and converting at the end. The answer in the book is in M, but I cannot even come close to that.

Any hints? I know for sure just plugging in grams of Br_2 and K_c, then solving for x doesn't get me even in the ballpark.

The book answer is 0.00767M. Which strangly enough is what I get what I convert 0.245g of Br_2 into M.

The answer I get at the end is 0.0028 M Br.