Which coefficients correctly balance the formula equation __ CaO __ H2O --> __ Ca(OH)2

The correct coefficients to balance this equation are 1, 1, and 1, which means the equation CaO + H2O → Ca(OH)2 is already balanced without needing any coefficients other than the implied "1" in front of each substance. When checking atom counts, you'll find one calcium atom on each side, two oxygen atoms on each side (one from CaO and one from H2O on the left; two in Ca(OH)2 on the right), and two hydrogen atoms on each side.

This reaction represents one of the simplest stoichiometric relationships in chemistry, where reactants combine in a perfect 1:1 ratio to form a single product. Many students initially expect to need coefficients because the formula Ca(OH)2 contains subscripts suggesting multiple atoms, but it's important to remember that subscripts within a formula are different from coefficients in front of formulas. The subscript "2" in Ca(OH)2 indicates there are two hydroxide groups bonded to one calcium atom, which perfectly accounts for the one oxygen from CaO and the one oxygen plus two hydrogens from H2O. This natural 1:1:1 balance makes the slaking of quicklime reaction straightforward to use in stoichiometric calculations for construction and industrial applications.