Empirical Formula Practice Test Questions

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The empirical formula of a compound represents the simplest whole-number ratio between the elements that make up the compound. This 10-question practice test deals with finding empirical formulas of chemical compounds.
A periodic table will be required to complete this practice test. Answers for the test appear after the final question:

Question 1

What is the empirical formula of a compound containing 60.0% sulfur and 40.0% oxygen by mass?

Question 2

A compound is found to contain 23.3% magnesium, 30.7% sulfur, and 46.0% oxygen. What is the empirical formula of this compound?

Question 3

What is the empirical formula for a compound containing 38.8% carbon, 16.2% hydrogen, and 45.1% nitrogen?

Question 4

A sample of an oxide of nitrogen is found to contain 30.4% nitrogen. What is its empirical formula?

Question 5

A sample of an oxide of arsenic is found to contain 75.74% arsenic. What is its empirical formula?

Question 6

What is the empirical formula for a compound containing 26.57% potassium, 35.36% chromium, and 38.07% oxygen?

Question 7

What is the empirical formula of a compound comprising 1.8% hydrogen, 56.1% sulfur, and 42.1% oxygen?

Question 8

A borane is a compound containing only boron and hydrogen. If a borane is found to contain 88.45% boron, what is its empirical formula?

Question 9

Find the empirical formula for a compound containing 40.6% carbon, 5.1% hydrogen, and 54.2% oxygen.

Question 10

What is the empirical formula of a compound containing 47.37% carbon, 10.59% hydrogen, and 42.04% oxygen?

Answers

1. SO3
2. MgSO3
3. CH5N
4. NO2
5. As2O3
6. K2Cr2O7
7. H2S2O3
8. B5H7
9. C2H3O2
10. C3H8O2
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Empirical Formula Tips

Remember, the empirical formula is the smallest whole number ratio. For this reason, it's also called the simplest ratio. When you get a formula, check your answer to make sure the subscripts can't all be divided by any number (usually it's 2 or 3, if this applies). If you're finding a formula from experimental data, you probably won't get perfect whole-number ratios. This is fine. However, it means you need to be careful when you're rounding numbers to make sure you get the correct answer. Real world chemistry is even trickier because atoms sometimes participate in unusual bonds, so empirical formulas aren't necessarily accurate.