How are chemical reactions represented?
This is the basis for any problem in stoichiometry. A relevant example is the combustion of hydrocarbons. Given the mass of an alkane, say
And if it ain't balanced (is it?) it's not in the race as a representation of physical (chemical) reality, because all chemical reactions are balanced with respect to mass and charge. Note that the equation as written is not only balanced with respect to mass and charge, it is also balanced with respect to energy. We could modify the equation to show how energy transfers in this reaction (how does it?). This also follows the principle of stoichiometry.
You always start with a balanced equation.
In a balanced equation, the coefficient in front of an element or compound is the number of moles of each element or compound.
The balanced equation for the decomposition of water by electrolysis is:
This tells us that two moles of water decompose to produce two moles of hydrogen gas and one mole of oxygen gas.