Calculate the partial pressure of ammonia in the container


Nitrogen gas reacts with hydrogen gas to form ammonia gas.  You have nitrogen and hydrogen gases in a 15.0-L container fitted with movable piston (the piston allows the container volume to change so as to keep the pressure constant inside the container).  Initially the partial pressure of each reactant gas is 1.00 atm.  Assume the temperature is constant and the reaction goes to completion.

  a)  Calculate the partial pressure of ammonia in the container after the reaction has reached completion.
  b)  Calculate the volume of the container after the reaction has reached completion.

ok first lets have a balanced chemical equation and start from there!


ok so what does dalton's law of partial pressure tell us about the total pressure in this problem?

The "initial" partial pressure is 2 atm (H 1 atm and N 1 atm) so the "final" pressure is going to be 2 atm.  Hydrogen is the limiting reactant so we'll end up with ammonia and excess nitrogen ( and the sum of their partial pressures is going to be 2 atm).  ???

dalton's law tells us that the total pressure exerted is the sum of the pressures that each gas would exert if it were alone, right???

ok so you know you can treat each gas individually but use the same volume so find the moles of reactants using PV=nRT

I did that: (1 atm)(15 L) = n(0.0821)(T); (I chose 273k for T) and I got n = 0.669 mol of Hydrogen. Then I  multiplied that number by 2/3 and got 0.446 mol of ammonia. 

Ohh I almost forgot, so after finding the number of moles I used PV = nRT to find the volume and I got    V = 10 L.  Is that the right answer??? If it is how can I now find the partial pressure of ammonia???

use the moles to find the final moles of NH3 and the moles of excess reactant

use that to get the mole fraction which will give you the partial pressure

P gas =  mole fraction of gas x total pressure

O.K. I did that, got a mole fraction of 0.5 and a partial pressure of 1 atm.

Got it Thanks!!!