Question

4 NH4NO3 -> 8 H2O + 4 N2 + 2 O2

What will be the total volume in liters of gas produced at 720. mm Hg and 570.0°C when 2.50 g of NH4NO3 completely decomposes? Use molar masses with at least as many significant figures as the data given.

Answer 1

Here is an example of a stoichiometry problem. Just follow the steps. You will need to do it three times, convert NH4NO3 each time to H2O, N2, and O2 and get a total number of moles. Then convert moles, using PV = nRT at the conditions listed, to volume. Here is the link.
http://www.jiskha.com/science/chemistry/stoichiometry.html

There is a shorter way. You CAN just do the calculation one time and use the moles from the first calculation to determine all of the others.

Answer 2

The easiest approach is to assume that all the gases are ideal gases, so that 4 moles of NH4NO3 produces 14 moles of gas.

so 2.5 g of NH4NO3 is 2.50/80.0 moles = 0.03125 moles

which will produce 0.03125 x 14/4 moles of gas = 0.1094 moles

which is 22.4 L x0.1094 = 2.45 L at STP

then use P1V1/T1 = P2V2/T2 to find the new total volume.

Alternatively use PV=nRT, where n=0.1094.