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Chemistry occurs in MOLE quantities. Masses (weights) are what we measure. So, to calculate reactions we need to put everything into terms of Moles before we can solve them. We also need to BALANCE the chemical reaction correctly. Then the ratio of moles of each substance can readily be seen.

The "limiting" reagent is simply the one that runs out first, and the "excess" is the one that is left over after the limiting reagent has been completely used up.

If the equation molar ratios are 2:3, and only 1 of the first is available, it is the limiting reagent, and it will be consumed when only 1.5 of the other is used. Thus, the second one is the "excess reagent" and will have 1.5 moles left in the solution after the first reagent is gone.

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