If the pressure of an N2 and H2 mixture in a closed vessel is 100 atm and 20% of the mixture reacts, what will be the final pressure at the same temperature?

Options:

• A. 100 atm
• B. 90 atm
• C. 85 atm
• D. 80 atm

Solution

Step 1: Write the balanced chemical reaction

N₂ + 3H₂ → 2NH₃

In this reaction, 4 moles of reactant gases produce 2 moles of product gas. This means the total number of gas molecules decreases during the reaction.

Step 2: Initial pressure

The total pressure of the gas mixture initially is 100 atm.

Since 20% of the mixture reacts:

20% of 100 atm = 20 atm

Step 3: Change in pressure due to reaction

From the reaction ratio:

• 4 parts reactants produce 2 parts products

Therefore, when 20 atm of reactant gases react, they form:

Product pressure = 10 atm

Step 4: Final pressure calculation

• Unreacted gases = 80 atm
• Ammonia formed = 10 atm

Pressure = 80 + 10 = 90 atm

Final Answer: 90 atm (Option B)