An organic molecule has the following mass percentages: 48.64% carbon, 8.16% hydrogen, and 43.20% oxygen. You are going to determine the empirical formula for this organic compound. Work out the problem in your notes. Attached are some questions to answer to verify you are solving the problem correctly.

How many moles of carbon are in the organic molecule? (Remember to use 4 significant digits in your answer.)

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How many moles of hydrogen are in the organic molecule? (Remember to use 3 significant digits in your answer.)

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How many moles of oxygen are in the organic molecule? (Remember to use 4 significant digits in your answer.)

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Since oxygen has the least amount of moles, you will need to divide each element's mole number by oxygen's moles.

Let's check the ratio numbers that you get after dividing each mole number by oxygen's moles.

C = 4.050 mol / 2.700 mol = 1.500 mol
H = 8.10 mol / 2.700 mol = 3.00 mol
O = 2.700 mol / 2.700 mol = 1.000 mol

Since carbon can't have a ratio with a decimal number, what is the smallest number that you can multiply 1.500 by to give you a whole number?

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You are correct. You can multiply 1.500 by 2 and get 3.000 as a whole number.

If you multiply carbon's ratio by 2, then you have to multiply each one of the element's ratio by 2. What is the empirical formula of this organic compound?

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If you would like another review, go to the accelerated Chem tutorial on Determination of Empirical and Molecular Formulas here.  Click on the image below to access the activity.

Be sure to click on the "Demonstration" buttons throughout.

Now you try it out! Determine the empirical formula from the percent compositions.

What is the empirical formula of the compound that has 22.1% Al, 25.4% P, and 52.5% O?

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What is the empirical formula of the compound that has 32.4% Na, 22.5% S, and 45.1% O?

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