Equilibrium: Final ClBr Concentration 0.15M/L

[Hockeystar]

Homework Statement

1 mol of each of the following gases is set up with the reaction K=4

IBr + Cl2 <-> ClBr + ICl

Find the final concentration of ClBr

Homework Equations

k= products/reactants

The Attempt at a Solution

First find Q to be 1 by finding the molarity of each gas to be 1/4 M/L. Q<K Therefore equilibrium must shift to right. Set up ICE table as follows:

4 = (1/4+x)²
--------------------
(1/4-x)²

x= 3.75?

nvm x should be = 0.15. Calculation mistake.

 

[Trave11er]

Hi Hockeystar!
Maybe it is easier to thing of the problem the following way: in the beginning you had 1 mol of each gas, then as K=4 some amount of reagents is used up and more of the products are formed.
If x mol IBr and x mol of Cl2 reacted to form more products then at equilibrium the amounts of gases will be these: 1+x mol of ICl and BrCl, and 1-x mol of IBr and Cl2, thus
K=(1+x)^2/(1-x)^2=4, x=0,33 (mol).

 

[Hockeystar]

But I thought when calculating Ksp the products and reactants must be molarity, not moles?

 

[Trave11er]

Actually it depends on the temperature of the reaction vessel. My assumption was that all the reactants and products are gaseous and thus it wouldn't matter whether you take molarities or quantities, because volumes cross out anyway.