# Coefficient in front of molecules in a chemical equation

In a chemical equation representing a chemical reaction what do the numbers in front of the molecules of compound and elements indicate? Do they signify number of molecules or moles or volume?

Example : $$\ce{ 2H_2 +O_2 -> 2H_2O}$$

Here does the "2" before $\ce{H2}$ and $\ce{H2O}$ indicate number of molecules or moles or volume? In a particular chapter in my textbook it was written that they indicate number of molecules(atoms) but in a later chapter the numbers were being treated as if they are number of moles or volume.

• 2 is the stoichiometric coefficient. It gives the relative amount of moles or volumes of reactants and products involved in the reaction. – user5764 Jun 30 '16 at 16:30
• Please don't call the 'amount of substance' 'number of moles', you wouldn't call the 'mass' 'number of kilo grams'. – Martin - マーチン Jul 2 '16 at 5:29