Ca(HCO3)2(s) decomposes at elevated temperatures according to the stoichiometric equation:
Ca(HCO3)2(s) → CaCO3(s) + H2O(g) + CO2(g).

Pure Ca(HCO3)2(s) is put into a sealed vessel, the air is pumped out, and afterwards the vessel is heated to 1200 K. Under these conditions, it is found that the pressure of the reaction mixture at equilibrium is 0.215 bar.
Determine KP under these conditions.

See explanation below

Explanation:

jejejeje to do this, we need to write again the reaction:

Ca(HCO3)2(s) → CaCO3(s) + H2O(g) + CO2(g).

We know that the partial pressure of any gas is expressed with the following formula:

Pi = Xi*P

Solving for P:

P = Pi/Xi

Now, as solid and liquids do not contribute in the equilibrium, we only see the gases and aqueous solutions. In this case, only gases are taken account in the expression of Kp.

If the total pressure of the reaction mixture is 0.215 bar at equilibrium, this means that the total pressure is made up of equal partial pressure of H2O and CO2, therefore our expression in equilibrium would be:

Kp = PH2O/P° * PCO2/P°

As I stated before, we have equal parts of pressure so PH2O = PCO2 therefore:

Kp = (PH2O/P°)²

Kp = (0.215/2)²  ---> the number 2 is because we have two gases there and 1 mole of each.

Kp = 0.01155