how do you write an expression for the equilibrium constant? and how do you calculate the equilibrium concentration of CO and H2? and the value of Kc

(ii) 0.5mol of gaseous methanol is placed in a 1L reaction vessel in the presence of an appropriate catalyst at elevated temperature and undergoes reaction until equilibrium is attained. At equilibrium, 0.1mol of unreacted methanol is present. Write out an expression for the equilibrium constant (Kc) in terms of the concentrations of reactants and products. Calculate the equilibrium concentrations of CO and H2 and work out a value for Kc