## Chem – Percentages

Percentages are very useful in chemistry. A percent is used in chemistry usually when we are trying to dissect something that has multiple parts. We describe each part as being a percent of the total. The percentage should always be the same even when we are dealing with a small or large amount of the same thing. Let’s go through an example.

Examples:

If a 100-box crate of ice cream is delivered to the ice cream store on a slow business day and 20% of the ice cream is vanilla, how many boxes are vanilla?

20% of 100 =

or

0.20 * 100 =

However, if a 200-box crate of ice cream is delivered to the ice cream store on a busy business day and 20% of the ice cream is vanilla, how many boxes are vanilla?

20% of 200 =

or

0.20 * 200 =

We can learn from this example that the amount of something (the number of boxes) does not affect the percentage. The percentage does not care if we have 100 boxes or 200 boxes or a million boxes. This gives us an advantage in some chemistry problems because that means we can use whatever amount we want because it will not affect the percentage.

The other thing worth noting in the examples is that you can convert a percentage to a decimal. In the first example, 20% and 0.20 are the same thing. This can be very helpful if you want to multiply or divide a percentage.

If you want to take a percent and turn it into a decimal, then you move your decimal point two spaces to the left. If you want to take a decimal and turn it into a percent, then you take your decimal point and move it two spaces to the right.

Examples: Give the decimal of the percent. VIDEO demonstration of the percentages and decimals below.

 45% 0.45 130% 1.3 5% 0.05 783% 7.83

PRACTICE PROBLEMS: Fill in the equivalent decimal or percentage.

 78% 0.78 36% 0.36 2% 0.02 96% 0.96 265% 2.65 9% 0.09  