When do know if you should ignore x or consider x when working out equilibrium shifts. I'm reading that if Keq is less than 1.0 hardly anything shift over to the product side, so x can be ignored. How do you know that hardly anything shifts over?

Here is an example of when x is considered:

At 92.2 C, the Kp for the following reaction is 0.2000 atm-1. If you were to place exactly 0.200 atm of N2O4 into a 1.00 L vessel, what would the partial pressure of NO2 be once equilibrium was established?

2NO2 --------> N2O4

A) 0.025 atm NO2

B) 0.200 atm NO2

C) 0.350 atm NO2

D) 0.400 atm NO2

Can anybody explain to me the logic in the explanation, without even working any math out for this problem? I probably can do this math, but that would take about five minutes to work out and of course the MCAT doesn't give us that time.

Here's the explanation: In this case Keq is less than 1 and the rxn starts with all products. The value of x is going to be sig. (more than half shifts over). If half of the 0.200 atm of N2O4 sifts over, than the partial pressure of nitrogen dioxide is 0.200 atm. Considering that more than half of the N2O4 is going to shift, the value of NO2 is greater than 0.200 atm. However, not all of the

N2O4 can shift over (which would result in 0.400 atm of NO2), so the answer must be less than 0.400 atm. Only choice C falls within the range of 0.200 and 0.400.