[khanbaba]

Consider the dissociation of NH3.
NH3 --> N2 + H2

balanced equation:
NH3 --> 1/2 N2 + 3/2 H2

At 350 K, 25 atm of NH3 decomposes and the total equilibrium pressure reaches 43 atm. At 400 K, 30 atm of NH3 decomposes and the total equilibrium pressure reaches 50 atm.
a. Calculate KP at each temperature.
b. Calculate ΔH,ΔS, and ΔG for the reaction.

Problem is that I dont know how to calculate the partial pressue of H2 and N2
the equation to use is Kp= ((Pn2)*1/2)((Ph2)*3/2) / Pnh3 

[nj_bartel]

The space between the molecules in a gas allows them to be treated differently than liquids and solids.  Disregard the molar mass differences of N2 and H2 - treat them using only mole fractions.

[khanbaba]

I dont get it, Like this is for environmental chemistry, so partial pressure can be assumed to be 0.8 for nitrogen and o.2 for oxygen, but then if you calculate the partial pressure of Hydrogen it comes out to be 17.2 which is too big I think.   

[nj_bartel]

Where'd oxygen come from?


When you're doing your calculations, are you taking into account that NH3 is disappearing?

[khanbaba]

there is no oxygen i was just giving an example, all im trying to do is that there is a total pressure given for each partial pressure of NH3. As sum of all pressure equals the total equilibrum pressure, I assumed that

PNH3 = 25 atm
PN2 = 0.8 atm (this is assumed for the atmoshere im not sure if this is correct).
Ptot = 43

Ptot= PNH3 + PN2 + PH2
hence, I calculated Ph2 to be 17.2 which i think is wrong. is there another equation i can use or a method i can use to calculate this.

thanks for your help

[nj_bartel]

Sorry, I was misreading part of the question.  In a bit of a hurry to get out right now, but disregard what I said earlier for the moment.  If someone hasn't replied by the time I'm available again I'll try to work it out.

[Yggdrasil]

Recall the ideal gas law, PV = nRT.  This rearranges to P = n (RT/V)

In a container with fixed volume and temperature, RT/V is a constant (lets call it C).  Therefore, P = nC (i.e. the partial pressure of a gas is directly proportional to the number of moles of that gas).

Now, lets say that 1 mol of ammonia gas in your container gives a pressure of 25 atm.  When the reaction is over, you have pressure of 43 atm, which would correspond to 1.72 moles of total gas molecules (ammonia + nitrogen + oxygen molecules).  How much of the original 1 mol of ammonia dissociated? (hint: make an ICE table)