[Big-Daddy]

The reaction of nitrogen gas and hydrogen gas yields ammonia. A closed vessel at 200C is filled with nitrogen gas 1.40 atm and hydrogen gas 2.60 atm. When the system is brought to equilibrium the total pressure become 2.40 atm. What is the partial pressure of hydrogen at equilibrium.

My method was to use equations P_eq(total)=P_eq(H2)+P_eq(N2)+P_eq(NH3) and then make substitutions for extent of reaction so that ε=1/2 * (P₀(H2)+P₀(N2)-P_eq(total)) where ε is the number of moles of reaction that went forward up until equilibrium. Therefore P_eq(H2)=P₀(H2)-3ε = P₀(H2)- 3/2 * (P₀(H2)+P₀(N2)-P_eq(total)) which I then evaluated to 0.20 atm. But for some reason this is wrong, and the answer was apparently 1.80 atm. Can anyone explain why?

[curiouscat]

I got 0.2 atm as well.

[Big-Daddy]

Ok, thanks for the reply, so I will assume it is ok to go with 0.2 atm unless someone else suggests an alternative or points out the mistake in my method here.

Here is another from the same exam which seems deadly simple and yet my answer refuses to concur with the given one. In fact this is almost too simple to think I could be wrong but maybe I've made an elementary error.

1. A sample of barium carbonate is contaminated x % of barium sulfate. A sample of 100.0g barium carbonate is reacted with excess hydrochloric acid and filtrated. The precipitate is evaporated to dryness.
The weight of the dried precipitate is 105.5g
( Atomic weight Ba  137.34 Cl   35.5 )
(1) Calculate the total volume ( at S.T.P ) of the gas evolved in the whole reaction process.
(2) Recalculate the volume of the gas at 1 atm, 27℃.
(3) Calculate x .

I got (1) 11.5 dm3, (2) 12.47 dm3, (3) x=0.06516%. Mark scheme gives (1) 11.2 dm3, (2) 11.94 dm3, (3) 1.33% (!). Someone is badly wrong on (3), either me or the question-writer. What do you think?

An interesting trivia is that this (IMO much easier) second question was worth 15 marks whereas the pressure question of the OP was worth 2! Trust the question-setters to get mark allocation just right :p