[Brian Lin]

At 5000 K and 1.000 atm, 83.00% of the oxygen molecules in a sample have dissociated to atomic oxygen. At what pressure will 95.0% of the molecules dissociate at this temperature?


Solutions reveal that you assume exactly 100 O2 molecules and make an ICE chart, giving you 17 O₂ molecules and 166 O atoms.

Then you find the partial mole fractions giving you X_o=166/183=0.9071 and X_O2=0.0929.

Then you have P_O2=X_O2P_total and  P_O=X_OP_total

What I don't get is that the solutions say that P_total= 1.000 atm, but should the initial pressure of the system (prior to dissociation) be DIFFERENT from P_total at equilibrium? MY reasoning is that moles is directly proportional to pressure and change number of moles will change initial pressure.

[Borek]

What I don't get is that the solutions say that P_total= 1.000 atm, but should the initial pressure of the system (prior to dissociation) be DIFFERENT from P_total at equilibrium?

Definitely. I guess 1.0 atm is the equilibrium pressure.