Chemistry · Basic concepts. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A balanced equation tells us the mole ratio in which substances react and form. Stoichiometry uses this ratio to find how much product a given amount of reactant can give. The route is always the same: change the given quantity to moles, use the mole ratio, then change the answer to mass or to volume at STP.
When two reactants are mixed, one usually runs out first. This is the limiting reactant. It decides how much product forms; the other reactant is left over (in excess).
The amount calculated from the equation is the theoretical yield. The amount really obtained in the laboratory is the actual yield, and it is nearly always smaller.
Students choose the reactant with the smaller mass, or the smaller number of moles, as the limiting reactant. Always compare the moles of product each reactant could make, because the mole ratio matters.
Answer: 75%. Magnesium is the limiting reactant and oxygen is in excess.
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