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The easiest way to do stoichiometric calculations involves using conversion factors. A conversion factor is a ratio (or fraction) which represents the relationship between two different units. A conversion factor is ALWAYS equal to 1. Here are some examples of conversion factors:
![]() ![]() ![]() |
As you can see it is extremely important to keep track of your units when using conversion factors. Without units, the first fraction would be 1 / 60. This is not equal to 1 and could very easily lead to wrong answers.
Furthermore, when you use units, you make it very easy to check your work. For example, perhaps you are trying to find out how many dozen eggs you have to buy to make three cakes. If you're getting an answer of 12 dozen eggs you might want to check your work. Could you even fit 12 of those cartons in your refrigerator? If you look back on your calculations you may immediately see the incorrect conversion factor: 1 egg / 12 dozen. It is easy to see that this is where the error occurred since this does NOT equal 1.
We all know from elementary school math that if you multiply any quantity by 1 you get the same quantity back. You can do this as many times as you want. For example, 2×1 = 2, and 18×1×1×1 = 18.
Multiplication by 1 is what you do whenever you do a problem involving
conversion factors. The best way to explain how to solve using
conversion factors is to work through some simple examples.
Problem: How many days are there in 3 years? (Assume none of these
years are leap years)
Solution: Here we basically want to convert years to days. Our
conversion factor is:
![]() |
3 years×![]() |
Conversion Factor = ![]() |
When doing any type of problem involving conversion factors, feel free to draw a line through any unit you see on the top and bottom of the fraction to make it visually obvious that the units cancel.
3 years×![]() |
Here's another, slightly harder problem: How many seconds are there in
3 years?
Solution: It is easiest to use multiple conversion factors for
this problem. Starting with the units you are given, find the
conversion factor needed to express to current unit in terms of the
next smaller unit.
3 years×![]() ![]() ![]() ![]() |
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