Conversions and Stoichiometry

Many questions in chemistry can be answered by thinking of them as conversions between units. This includes stoichiometry questions. We'll get into what stoichiometry means in a bit, but first let's look at solving questions as conversions.

To solve questions as conversions, you need to have conversion factors. Conversion factors relate units to one another. For example, there are 100 cents in 1 dollar, or 100 cents = $1. To use this as a conversion factor, write it as a fraction:

Since 100 cents = 1 dollar, this fraction is equal to 1. That means you can multiply it by anything else, and still have the! same quantity that you started with. For example, if you want to convert $5.27 to cents, you can multiply it by the fraction. The dollars cancel out, leaving you with 525 cents:

We can also flip the fraction to get a new conversion factor:

As long as the top and bottom are equa! l to one another, and we're careful to cancel out units to see! what we have left, we can string conversion factors like this together to convert from a starting number to a final answer.

That brings us to stoichiometry. Stoichiometry is the study of relationships in chemical reactions. in other words, it's the study of what amounts of things are equal to amounts of other things in chemical reactions.

For example, say you have a chemical reaction like this (sorry for the formatting, I may make it prettier for you later):

2A --> 3B

(where A and B are elements or compounds)

That chemical reaction tells us that each time 2 mol A reacts, 3 mol B are produced. In other words, for this reaction, 2 mol A = 3 mol B. We can use that equivalency to make conversion factors!

To answer stoichiometry questions, you usually only need two more pieces of information: the molar masses of the reactants and/or products involved in ! the question (usually denoted M or with the script M shown below), and the definition of a mole. We won't cover molar masses here (see the Wikipedia article on molar masses if you need help), but we'll remind you that 1 mol = 6.022 × 10²³ things (atoms or molecules or whatever). You might find this figure helpful to sort out how all of those conversion factors fit together (I know it's confusing; I'll try to be back with an easier-to-follow version soon!):

To convert from a given amount to a target amount, start at the given amount, and follow the arrows until you get to the target amount. S! tring the conversion factors together (switching the numbers t! o match your particular problem), and you should get your target answer. Check that the units cross out properly to make sure!

Work through the question below to see if you understand how to answer stoichiometry questions.

Try a new version of this question.

If you need more practice with questions involving stoichiometry and chemical reactions, sign up for the Self-Study Course in Chemical Reactions at Sapling Learning. Each question has a ful! l, in-depth solution and specific feedback to help you identify your mistakes.

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