The decomposition of calcium carbonate will follow
CaCO3(s) --> CaO(s)+CO2(g)
For this reaction Kp= PCo2 because others are solids.
So we need to find the pressure of carbon dioxide
It is given that 30% of 20g CaCO3 remains unreacted so 70% must be reacted
70% of 20g = 14g
Number of Moles of CaCO3= 14g/100gmol-1(Molar mass of CaCO3=100g)
= 0.14moles
In according to the equation we will get 0.14 mole of CO2
Pressure of CO2 can be found using ideal gas equation
PV=nRT
Px10 litre = 0.14moles x 0.082 x (800+273)
Which will give pressure as 0.14x0.082x1073/10 = 1.23 atm
This is the Kp of equilibrium.