Question

one mole of HI introduce in vessel held at constant temperature when equilibrium is reached it is found that 0.1 mole of I2 have been formed .calculate the equilibrium constant kc

Answer 1

I'm assuming the 1 mole is in a 1 L container.
...................2HI(g) ===> H2(g) + I2(g)
I....................1..................0............0
C..................-2x...............x............x
E...................1-2x.............x............x

Kc = (H2)(I2)/(HI)^2.
So you know I2 = 0.1 Mole/L which makes (H2) = 0.1 mols/L and it makes HI = (1-0.2).
Substitute those values into Kc expreession and solve.
NOTE: Concentration in moles/L is what goes into the Kc expression; therefore, technically, we can't calculate Kc until the volume in known. That's why I've assume a volume of 1 L.