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Q.20. If the pressure of N 2   a n d   H 2 mixture in a closed apparatus is 100 atm and 20 % of the mixture reacts then the pressure at the same temperature would be -
(1) 100
(2) 90
(3) 85
(4) 80

Dear Student,

The equilibrium reaction will be:                                   N2 + 3H2   2NH3initial pressure         p           3p             0at equilibrium       p-x      3p-3x         2xAs we know, the total pressure = 100atmi.e. p+3p=100; 4p=100; p=25atmAs we know 20% mixture reacts, αAcc. to the formula, α = x/p; x=5Initial pressure of N2 = p = 25atmInitial pressure of H2 = 3p = 3*25 = 75atmAt equilibrium, pressure of N2 = p-x = 25-5 = 20atmpressure of H2 = 3p-3x = 75-15 = 60atmpressure of NH3 = 2x = 10atmTotal pressure = 20 + 60 + 10 = 90atm 
Therefore option (2) 90 is the correct answer.

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