**Law of Mass Action**

For a general reaction of the form [itex]aA +bB \rightarrow cC +dD [/itex], the law of mass actions states that the equilibrium condition is expressed by the equation,[itex] \frac {C^c \cdot D^d}{A^a\cdot B^b}[/itex]

Now, for the reaction [itex] H_2(g) + I_2(g) \rightleftharpoons 2HI (g)[/itex], initial concentrations of Hydrogen molecule and Iodine molecule as per experiment (1) 0.024M and 0.0138M respectively. Let X mole/liter of each of the product be formed.

At equilibrium, the concentrations would be [itex]H_2=[0.024M-X], I_2=[0.0138-X], 2HI=[X \cdot M] [/itex]

So, [itex]K_c=\frac{X^2}{(0.024-X)(0.0138-X)}=46.42 \Rightarrow X^2=46.42(X^2-0.0378 \cdot X +0.0003312[/itex]

Now, solving for X, we get X=1.3432205555e-2 or X=2.52000269414e-2.

Now X=1.34322e-2 do not match with 0.0252 M of 2HI. So X=0.0252 M of 2HI.

So, equilibrium concentration of [itex]H_2[/itex] is 0.024M - 0.0252 =-0.0012 M which is actual equilibrium concentration of [itex]I_2[/itex] with minus sign. Does this mean it is the equilibrium centration of Iodine molecules?

and,

equilibrium of concentration of [itex]I_2[/itex] is 0.0138 M- 0.0252 M=-0.0114M which is actual equilibrium concentration [itex]H_2[/itex] with minus sign. Does this mean it is the equilibrium concentration of Hydrogen molecules?

Why we should neglect the first value of X?