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Step 1:
I'll solve this problem step by step, carefully following the LaTeX formatting guidelines:

Step 2:
: Write the lattice energy equation

The lattice energy equation for $$\mathrm{MgCl}_{2}(s)$$ can be represented as:
\mathrm{Mg}^{2 +}(g) + 2\mathrm{Cl}^{-}(g) \rightarrow \mathrm{MgCl}_{2}(s)

Step 3:
: Identify the given thermochemical data

- Formation reaction: $$\mathrm{Mg}(g) + 2\mathrm{Cl}(g) \rightarrow \mathrm{MgCl}_{2}(s)$$, $$\Delta H = -642 \mathrm{~kJ/mol}
- Chlorine electron affinity: \mathrm{Cl}(g) + e^{-} \rightarrow \mathrm{Cl}^{-}(g), \Delta H = - 337 \mathrm{~kJ/mol}

Step 4:
: Calculate lattice energy using Hess's Law

\text{Lattice Energy} = -\Delta H_{\text{formation}} - \Delta H_{\text{ionization}} - 2\Delta H_{\text{electron affinity}}
Rearrange the formation equation to solve for lattice energy:

Step 5:
: Substitute the values

\text{Lattice Energy} = -(-642) - (+2188) - 2(-337)

Step 6:
: Calculate the lattice energy

\text{Lattice Energy} = -872 \mathrm{~kJ/mol}
\text{Lattice Energy} = + 642 - 2188 + 674

Final Answer

The lattice energy of \mathrm{MgCl}_{2}(s) is - 872 \mathrm{~kJ/mol}.