The statement for the leftover reactant is "two molecules of O2."
What is leftover reactant?
Excess reagents are reactants that are not consumed fully when a chemical reaction is complete. After a chemical reaction has reached equilibrium, the reactant(s) remain. The excess reagent is the reactant that is also referred as "left over reactant" in a chemical reaction.
The chemical structure formed when two or more atoms chemically link is known as a molecule. When molecular hydrogen and oxygen are mixed to react with each other, energy is released in the process, resulting in the formation of a water molecule.
2H2 + O2 → 2H2O
That is, two molecules of diatomic hydrogen (H2) must be joined with one molecule of diatomic oxygen to form two molecules of water (H2O).
So, leftover reactant will be two molecules of O2, which remains as reaction gets completed.
Hence the correct answer is two molecules of O2.
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