Calculate the heat of formation of `MgCO_2` (s) from the following data
`C("graphite") + O_2(g ) to CO_2(g) , DeltaH = -393.5 kJ`
`2Mg(s) + O_2(g) to 2MgO(s) , DeltaH = -1203 .6 kJ`
`MgO (s) + CO_2(g) to MgCO_3 (s) , DeltaH = - 117.7 kJ `

Explanation

To calculate the heat of formation of \( \text{MgCO}_3 \) from the given data, we can use Hess's law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction.

### Given Reactions:
1. \( \text{C (graphite)} + \text{O}_2(g) \rightarrow \text{CO}_2(g) \)
\( \Delta H = -393.5 \, \text{kJ} \)

2. \( 2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s) \)
\( \Delta H = -1203.6 \, \text{kJ} \)
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