Chapter 9
Calculations from Chemical Equations

Chapter 9 gives us a new conversion factor for chemical calculations. It's the mole-to-mole conversion for calculating the amounts (moles, atoms, or molecules ---not grams!) of reactants or products that participate in a chemical reaction.

We already know how to convert:

moles-to-grams
This conversion uses the molar mass of an element or compound as the conversion factor. The units for molar mass are grams per mole, just what you need for converting grams to moles or moles to grams.
moles-to-atoms (or moles-to-molecules)
This conversion uses the definition of a mole. There are 6.022 x 1023 things in a mole of things. This is true for any "thing." The units for Avogadro's number are things per mole. Substitute atoms or molecules for "things" and there you are.
molecules-to-atoms
This conversion uses the subscripts in a molecular formula. That's what the subscripts are there for, to tell you how many atoms of each element are in a molecule.

Here's the new conversion factor:

moles-to-moles
That's moles of something to moles of something else. Those "somethings" are reactants or products in a chemical reaction. If you know how much of one reactant reacts, then you can calculate how much of the other reactants must react (and how much is left over, if any), and how much of each product is produced.

If you know how much (moles) of one product is produced, you can calculate how much of the other products are produced and how much of each reactant must react.

To do these conversions, you use a mole-to-mole conversion factor from the coefficients of your BALANCED chemical equation. If it ain't balanced, this ain't gonna work at all.

If you know the mass of a reactant or product, you can't just use the coefficients to convert to grams of something else! First, convert from grams to moles (see above), then do the mole-to-mole conversion.

Limiting Reactant Calculations

The calculations described above are called "stoichiometric" calculations. They tell us the amounts (moles, masses, etc.) of an element or compound that react or are produced.

Sometimes you don't have the correct ratios of reactants. You have too much or something and too little of something else. Actually, this is usually the case.

Mole-to-mole conversions tell you how much will react. If there's too much of one of the reactants present, the mole-to-mole conversions will tell you how much will react and how much will be left over. It's just that simple.

If you're given the amounts (grams or moles, or even liters) of each reactant, then you calculate the amount (moles) of product that each reactant would produce if it reacted completely. Then you take the smallest amount of product as your real answer.

The reactant that produces the smallest amount of product is the limiting reactant. The others are present in excess, and can't react all the way. There will always be some left over.

Use the limiting reactant to calculate how much of each product is produced and how much of each reactant reacts..

Yield Calculations

If you are doing an experiment and have a certain amount of each reactant, you can calculate the amount of each product that you would expect to be produced. See above for how to do this. The amount you expect is the "theoretical yield." The amount you actually get is the "actual yield" (think about it). The percent yield is the actual yield divided by the theoretical yield, times 100.

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