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**We are surrounded by percentages.** The tag inside of our t-shirt tells us what percentage of our t-shirt is made up of cotton. The nutrition facts on the side of a box of cereal tells us the daily percentage of nutrients per serving. The results of your last test in chemistry class were given to you in as percentage. **A percent means “for every 100” or “out of 100.”**

How do you calculate a percent? Let’s consider your last chemistry. If you obtained 82 correct answers and there were 100 questions on the test. You got 82% of the questions correct. Here’s the math:

This example was pretty simple but every other percentage is calculated in the same way.

In chemistry we are often concerned with the composition of a compound. We can express the relative amounts of elements within a compound by calculating the **percent composition.** The percent composition of a compound compares the masses of each individual element to the mass of the whole compound. **To calculate the percent composition of a compound we divide the mass of the element by the total mass of the compound and multiply by 100 to turn this quantity into a percent.**