7.17 Kp = 0.04 atm at 899 K for the equilibrium shown below. What is the equilibrium concentration of C2H6 when it is placed in a flask at 4.0 atm pressure and allowed to come to equilibrium?

C2H6 (g) C2H4 (g) + H2 (g)


Let p be the pressure exerted by ethene and hydrogen gas (each) at equilibrium. Now, according to the reaction,

                                C2H6g           C2H4g     +    H2gInitial conc.             4.0 atm                    0                    0At equilibrium         4.0-p                     p                   pWe can write,pC2H4×pH2pC2H6=Kpp×p40p=0.04p2=0.160.04pp2+0.04p0.16=0Now,    p=0.04±(0.04)24×1×(0.16)2×1=0.04±0.802=0.762      Taking positive value=0.38

Hence, at equilibrium,

C2H6 = 4-p=4-0.38                         =3.62 atm