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Homework Statement:

Below is the reaction for the industrial synthesis of acrylonitrile (C3H3N) from propene (C3H6):
A 150.0 liter steel reactor at 25 ºC is filled with the following partial pressures of reactants: 0.500 MPa C3H6, 0.800 MPa ammonia, and 1.500 MPa oxygen gas.
The reaction equation that describes the process is as follows
2 C3H6 (g) + 2 NH3 (g) + 3 O2 > 2 C3H3N (g) + 6 H2O (g)
If the reaction described in question part a proceeds to 90% yield, and no side reactions take place, what is the final total pressure in the reaction vessel at 400 ºC?
Relevant Equations:
 PV = nRT ; Total Pressure = P1 + P2 + ... P(n)
I had already found the Mass of the product (C3H3N) produced by this reaction (theoretical mass at 100% yield) in a previous problem. I did this by finding the Limiting Reagent (C3H6) in the reaction , calculating the number of moles of C3H6 and using the Molar Ratios in the balanced reaction equation to find the number of moles of product produced (C3H3N) and so from this .. calculating the product Mass, and that Mass was found to be 1603.8g (1.604Kg).
Now to solve this problem as described in Homework Statement section, do I now calculate the mass of the other product (H2O) and then get the number of moles of both these products and of the nonlimiting reactants (NH3 and O2) then from here apply the Ideal Gas Law to find the Partial Pressures of each of the nonlimiting reactants and products (NH3; O2; C3H3N and H2O) sum all of these Partial Pressures to get a Total final Pressure value then finally multiply the final sum of these pressures by 0.9 (ie. 90%) to find the correct total pressure ?
Is this correct or am I on the wrong track ? If so is there some alternative way of tackling this problem ?
Now to solve this problem as described in Homework Statement section, do I now calculate the mass of the other product (H2O) and then get the number of moles of both these products and of the nonlimiting reactants (NH3 and O2) then from here apply the Ideal Gas Law to find the Partial Pressures of each of the nonlimiting reactants and products (NH3; O2; C3H3N and H2O) sum all of these Partial Pressures to get a Total final Pressure value then finally multiply the final sum of these pressures by 0.9 (ie. 90%) to find the correct total pressure ?
Is this correct or am I on the wrong track ? If so is there some alternative way of tackling this problem ?