Sometimes you may need to determine the number of particles of a certain element in a compound. In order to do this, you must first know how many atoms of that element are in the compound.

For example, how many atoms of each element are in one molecule of H₂O?

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Hydrogen = 2 atoms, Oxygen = 1 atom

How many atoms of each element are in one molecule of Al₂(SO₄)₃?

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Aluminum = 2, Sulfur = 3, Oxygen = 12

Now, let’s look at a practice problem that requires you to put this information together with the information you learned in the previous section.

Practice Problem 1

How many Al ions are present in 2.5 moles of Al₂(SO₄)₃?

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Practice Problem 2

How many N ions are present in 17.5 moles of Ca(NO₃)₂?

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