How to Find the Number of Atoms in an Individual Element in a Compound

Every 8 ounce glass of water contains 7.5 * 10^24 molecules or compounds of H2O.

Compounds like water, baking soda, marble and salt are all around us. They're collections of two of more elements whose atoms are chemically bonded. Take the compound water, for instance. It is composed of two hydrogen atoms and one oxygen atom. But this doesn't mean that the water in your cup has just two hydrogen atoms and one oxygen atom. The number of actual atoms per each element is based on how much of the compound you have. Once you know this, you can solve for the number of atoms in an individual element in a compound.

Instructions

1. Determine the amount of compound you have in moles, a unit of measurement equivalent to 6.022 * 10^23 molecules of a substance. Keep in mind that a compound is simply a molecule with two or more elements that have been chemically bonded together.

2. Establish the compound's formula. Given you are trying to measure the compound water, for instance, you would use the formula H2O. This formula specifies that every molecule of water contains two hydrogen atoms and one oxygen atom.

3. Determine the number of atoms per mole of the compound. Remember that 1 mole is equal to 6.022 * 10^23 molecules of a substance. Given you are measuring water, you know each molecule has two hydrogen atoms and one oxygen atom. Since there are 6.022 * 10^23 molecules of water in 1 mole of water, you know each mole of water contains (6.022 * 10^23) multiplied by 2 hydrogen atoms, and (6.022 * 10^23) water atoms.

   Hydrogen atoms per mole of water: 2 * (6.022 * 10^23) = 1.204 * 10^24

   Oxygen atoms per mole of water: (6.022 * 10^23) = 6.022 * 10^23

4. Multiply the values obtained in Step 3 by the value obtained in Step 1. Given you have 1000 moles of water and you know each mole of water contains 1.204 * 10^24 hydrogen atoms and 6.022 * 10^23 oxygen atoms, you know the total number of hydrogen atoms is 1000 * (1.204 * 10^24) and the total number of oxygen atoms is 1000 * (6.022 * 10^23):

   Total number of hydrogen atoms in 1000 moles of water: 1000 * (1.204 * 10^24) = 1.204 * 10^27

   Total number of oxygen atoms in 1000 moles of water: 1000 * (6.022 * 10^23) = 6.022 * 10^26

References

• Michigan State University: Counting Atoms
• Austin Community College: The Mole Concept and Chemical Formulas