[herob4u]

What is the situation in which you simply CAN'T directly relate mole numbers through stoichiometric ratio of a chemical equation; what is the intuition behind it. I find myself confused over these simple topics recently, especially when dealing with gases. In a lot of cases, I was just not "allowed" to do direct conversion, and I never understood WHY.

An example is below:

Under certain conditions, the well-known fertilizer NH4NO3 (s) explodes according to the
following balanced reaction:
NH4NO3 (s) --> N2 (g) + 2H2O(g) + 0.5O2 (g)
Exactly 2 g of NH4NO3 is placed in a 8.26 L, constant volume container containing air at 76.47
kPa and 15 ºC. The explosive reaction is complete and the temperature rises to 532 ºC. The
container remains intact after the explosion. Determine the mole fraction of N2 in the gas phase
after the explosion.
Data: Assume that the composition of air is 79% N2 and 21% O2 by volume.

I do NOT necessarily require the solution to the problem. I merely want to know what is the difference between the number of moles you get using PV = nRT, vs direct stoichiometry. How do I become more AWARE of this.

[Borek]

Amount of N₂ produced in the decomposition is given by the stoichiometry. Total amount of nitrogen must take into account initial nitrogen from the air that filled the vessel before the reaction started.

[Irlanur]

explodes

That's a point where the stochiometry can become extremely comlicated. If you just have a mess of complete/incomplete reactions.

PV = nRT

is the equation for an ideal gas. most gases are not ideal.