What is the molar quantity of carbon dioxide if #0.75*mol# propane gas is combusted completely?
A stoichiometrically balanced equation is an absolute prerequisite.
Is this equation stoichiometrically balanced? That is, for every reactant particle, is there a corresponding product particle? In fact there is, and there must be. You might think this is a bit unfamiliar, but if you were receiving change from a cash purchase, you would be able to tell whether you were being short-changed immediately.
Given this, the equation unequivocally tells us that for each mole of propane,
The equation as written was for 1 mole of propane. If there are
See here for another problem illustrating conservation of mass.