Question

We mix 400. torr of H2 and 100. torr of N2 in a 2.00 liter flask at 400 C. Assume that the following reaction occurs and goes to completion :

N2 (g)+ 3H2(g) -----> 2NH3 (g)

A) What is the limiting reagent ? Show by calculation

B) After the reaction has been completed, what pressure of each gas is present in the flask and what is the total pressure ?

Could you write down the step by step processing for me please. Thank You.

Answer 1

As the volume and temperature is constant for the container we can say that the pressure is directly proportional to the moles of the gases as by ideal gas equation we have PV = nRT where P, V,n,T is the pressure, volume, moles and temperature of the gas respectively and R is the gas constant.

P = nRT/V

So, if volume and temperature is constant we have

P = kn where k is RT/V which is a constant

or n = GP where G is 1/k

so, moles of N2 = G*100 moles

and moles of H2 = G*400 moles

From the balanced chemical equation we have -

N2 + 3H2 -> 2NH3

So, each mole of N2 reacts with 3 moles of H2 to give 2 moles of NH3

But we have

G*100 moles of N2 so, they will react with only 3*G*100 moles of H2 = G*300 moles of H2 to produce 2*G*100 moles of NH3

So, H2 is in excess as H2 = G*400 moles

So, N2 is the limiting reagent.

Finally after reaction we will have

0 moles of N2 , G*400 moles - G*300 moles = G*100 moles of H2 and G*200 moles of NH3

so, total pressure of the gas can be given as

Ptotal * V = ntotal*R*T ----- Eqn1

ntotal = nH2 + nNH3 (nH2 = no. of moles of H2 and nNH3= moles of NH3)

= G*100 + G*200 = G*300

Also, G = 1/k = 1/(RT/V) = V/RT

Putting Values of G and ntotal in the Eqn1 above we get

Ptotal * V = (300*V/RT)*R*T

So, Ptotal = 300 torr

Also,

pressure of H2 (pH2)*V = (nH2)*R*T

Where nH2 is the no. of moles of H2

so, applying nH2 = G*100 = 100V/RT we have

pH2 * V = (100*V/RT)*RT

pH2 = 100 torr

so, pNH3 = Ptotal - pH2

pNH3 = 300 torr - 100 torr

= 200 torr