Question of Chemical Equilibria

Pure hydrogen iodide (HI) is a gas, which at high temperatures, partially dissociates into hydrogen and iodine according to the equation:

2HI (g) < —- > H2(g) + I2(g)

At 500K, the equilibrium constant, Kc, for the dissociation reaction is 6.25 x 10^-3.  Some pure HI is placed into an evacuated 2.0 dm3 glass tube and heated to 500K. In the equilibrium sample, the concentration of I2 is 3.10 x 10^-5 moldm^-3.

ai) Write an expression for the Kc for the dissociation of HI.
ii) What are the concentrations of H2(g) and HI(g) in this equilibrium mixture at 500K?
iii) How many moles of HI  must have been placed into the 2.0dm3 glass tube originally?

b) At 600K, Kc for the dissociation of HI is 1.56×10^-2. Describe and explain the appearance of the contents in this glass as it is heated to 600K.

c) Hydrogen chloride at 500K undergoes no significant dissociation into its elements. Any dissociation of hydrogen chloride can be completely ignored. What is the pressure in a 750cm3 flask containing 8.20 x 10^-2 mol of pure HCl at 500K?

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