For the equilibrium
`MgCO_3(s)oversetDeltahArrMgO(s)+CO_2(g)` which of the following expressions is correct ?

Options

A. `K_p=pco_2`
B. `K_p=([MgO][CO_2])/([MgCO_3])`
C. `K_p=(p_(Mgo).p_(CO_2))/(p_(MgCO_3))`
D. `K_p=(p_(Mgo)+p_(CO_2))/(p_(MgCO_3))`

Explanation

To solve the question regarding the equilibrium reaction:

\[ \text{MgCO}_3(s) \rightleftharpoons \text{MgO}(s) + \text{CO}_2(g) \]

we need to determine the correct expression for the equilibrium constant \( K_p \).

### Step-by-Step Solution:

1. **Identify the Reaction Components**:
The reaction involves magnesium carbonate (\( \text{MgCO}_3 \)) in solid form, magnesium oxide (\( \text{MgO} \)) also in solid form, and carbon dioxide (\( \text{CO}_2 \)) in gaseous form.

2. **Write the Expression for \( K_p \)**:
The equilibrium constant \( K_p \) is defined in terms of the partial pressures of the gaseous products over the reactants. The general form for \( K_p \) is given by:

\[ K_p = \frac{P_{\text{products}}}{P_{\text{reactants}}} \]

For our reaction, this becomes:

\[ K_p = \frac{P_{\text{CO}_2}}{P_{\text{MgCO}_3} \cdot P_{\text{MgO}}} \]

3. **Consider the States of Matter**:
In this equilibrium, both \( \text{MgCO}_3 \) and \( \text{MgO} \) are solids. The key point is that the concentrations (or partial pressures) of solids do not appear in the equilibrium expression because they remain constant and are essentially considered to have a value of 1.

4. **Simplify the Expression**:
Since the concentrations of solids do not affect the equilibrium constant, we can simplify the expression:

\[ K_p = \frac{P_{\text{CO}_2}}{1 \cdot 1} = P_{\text{CO}_2} \]

5. **Conclusion**:
Therefore, the correct expression for \( K_p \) for the given equilibrium is simply:

\[ K_p = P_{\text{CO}_2} \]

This means that \( K_p \) is equal to the partial pressure of carbon dioxide.

### Final Answer:
The correct expression for the equilibrium constant \( K_p \) for the reaction is:

\[ K_p = P_{\text{CO}_2} \]