Unit Analysis

In science, it is not uncommon to change from one system of measurement to another. The best method to accomplish this change is unit analysis.

In science calculations, the units are very important and must be treated in the same way as constants and variables are treated in algebra.

Notice in the example how the units multiply, divide and cross cancel just like numbers and variables.

Conversions

Science calculations require close examination of the units. Therefore, you must know the conversion factors that allow you to make the desired calculations.

Common Conversion Factors

conversion factors

This Per That Problems

The typical type of chemistry problem is a "this per that" or what is called a conversion problem. An example of a this per that would be 12 inches per 1 foot.

The hardest part of a this per that is knowing the conversion factor (this per that). Once you know the conversion factor all you have to do is set up the equation, verify your units, and plug&chug (put it in the calculator).

Example

There are 500 monkeys and you can put 5 monkeys in a cage.
How many cages will you need?

500 monkeys        5 monkeys per cage        How many cage?

Reasoning tells me that if I divide 500 by 5 I will need 100 cages.

this per that

The above example is a this per that problem and similar in logic to the types of problems you will do in chemistry. So, let's look at it set up like a chemistry problem.

this per that

 Note that monkeys cancel out and cages is our new unit.