Problem: Given the data in the table on the right, the ΔH°rxn for the reaction:Ca(OH)2 + 2 H3AsO4 → Ca(H2AsO4)2 + 2 H2O  is ___________ kJ.A. -744.9B. -4519C. -4219D. -130.4E. -76.4

We are asked to determine the ΔH°_rxn for the reaction: 

Ca(OH)₂ + 2 H₃AsO₄ → Ca(H₂AsO₄)₂ + 2 H₂O 

We can use the following equation to solve for ΔH˚_rxn:

$\boxed{\mathbf{\Delta H}{\mathbf{°}}_{\mathbf{rxn}}\mathbf{=}\mathbf{\Delta H}{\mathbf{°}}_{\mathbf{f}\mathbf{,}\mathbf{ }\mathbf{prod}}\mathbf{-}\mathbf{\Delta H}{\mathbf{°}}_{\mathbf{f}\mathbf{,}\mathbf{ }\mathbf{react}}}$

Note that we need to multiply each ΔH˚_f by the stoichiometric coefficient since ΔH˚_f is in kJ/mol. 

Also, note that ΔH˚_f for elements in their standard state is 0.