# Homework Help: Kp problem with total pressure given

1. Nov 15, 2009

### marquitos

For the equilibrium shown below, Kp = 0.20 at 298K
PCl5(g) <--> PCl (g) + Cl2 (g)
A certain amount of PCl5 is added to a 1.0-L flask. When equilibrium is reached, the total pressure in the flask is 3.00atm. What is the initial pressure of PCl5 in the flask?

If someone could point me in the right direction I'm having extreme difficulty even starting the problem, in class we haven't gone over anything like this
Thank you

2. Nov 15, 2009

### Staff: Mentor

Do you know how to use ICE tables?

Basically use stoichiometry of the raction to combine amounts of the substances present at equilibrium with total amount and Kp.

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3. Nov 15, 2009

### marquitos

we have done ICE tables but for this reaction none of the initials are given how would I find them with the given information?

4. Nov 15, 2009

### Staff: Mentor

You will have two variables - x (as usually in ICE tables) that refers to the amount of substances that reacted, and y - which is initial amount of PCl5. Everything else is initially at 0.

Trick is - this is not enough information to solve, but amounts of substance - when combined with known volume, temperature and pressure - will give you second equation. That means two equations in two unknowns.

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