I need help answering this physical chemistry question!

hawa · Rookie Joined: 19 Jan 2010 Posts: 1

Hi everyone Very Happy

This is a GAMSAT practice question and goes as follows

A gas X dissociates on heating to set up the equilibrium
X(g) ⇌ Y(g) + Z(g)
A quantity of X was heated at constant pressure p atm to a certain temperature where the equilibrium partial pressure of X was found to be p/7atm.
What is the equilibrium constant Kp at this temperature?
A. 6p/7
B. 9p/7
C. 6p
D. 9p

I need an explanation of how to reach the answer to this question

pyro · Rookie Joined: 30 Apr 2009 Posts: 4

Hello my friend,

Now the equation is balanced and the partial pressure of X is p/7 while the total pressure is p....so far so good..happy happy joy joy...
step 1: if the partial pressure of X is p/7 we can now find the partial pressure of the two other gases namely Y and Z...you may ask how...

partial pressure of X is p/7 so partial pressure of Y and Z combined is (Total pressure- partial pressure of X) = p-p/7 = (7p-p)/7 = 6p/7

partial pressure of Y = (6p/7)/2= 6p/7 x 1/2 = 3p/7

Step 2: Kp expression for the given equation = [p(Y) x p (z)]/ p(x)...now plug in the values...wee...wee...a bit of French vibe with a pinch of cocaine..sniff....[(3p/7) x (3p/7)]/(p/7) = 9p/7......Please shoot me if I am wrong....please...please...

129west81st · Regular Joined: 15 Oct 2009 Posts: 27

So... let me just clarify... to find K in an ideal gas situation, you just divide products/reactants?

prepgenie · Grizzled Joined: 08 Dec 2008 Posts: 113

Yes that's right. In case of an ideal gas, the equilibrium constant (Kp) for the following reaction involving all gaseous compounds:

can be given by-

where

are the number of moles of the reactants A & B and the products S & T respectively and

are the partial pressures of A, B, S and T respectively.
Thus, for the given equation: X(g) ⇌ Y(g) + Z(g), the equilibrium constant can be given by: