Final answer:
Without the specific data for 900°C, such as CO₂ concentration at equilibrium, we cannot accurately provide the Kp for the decomposition of CaCO₃ at that temperature.
Step-by-step explanation:
The equilibrium constant for the decomposition of CaCO₃(s) into CaO(s) and CO₂(g) can be represented as Kp = [CO₂]. At 800°C, the concentration of CO₂ in equilibrium is given as 2.5 × 10⁻³ M, and so the equilibrium constant K at 800°C is also 2.5 × 10⁻³. Given that the question asks for the equilibrium constant at 900°C instead, but does not provide the necessary data for that specific temperature, we cannot accurately determine the equilibrium constant at 900°C without additional data, like CO₂ equilibrium concentration at 900°C or a Van't Hoff plot.