Skip to content

HS Chemistry - Working With Chemical Reactions

Moles of Particles

Overview of The Page

This page will cover:

  • What are moles?
  • Determining the number of moles of a substance.

Particles like atoms, ions, and molecules are too small to be counted individually. How can we know how many there are in a substance?

Moles

A mole is a counting unit, not a unit of mass. Therefore, one mole of oxygen molecules has the same number of molecules as one mole of chlorine molecules. One mole of hydrogen atoms has the same number of atoms as one mole of uranium atoms. One mole of sodium ions has the same number of ions as one mole of chloride ions.

1 mole of something = 6.02 × 1023 of that thing for example, 1 mole of Hydrogen atoms is 6.02 × 10^23^ Hydrogen atoms. 6.02 × 1023 amu = 1 gram. Therefore, if 1 atom of carbon-12 has 12 amu, 1 mole of carbon-12 has 12 grams. For this reason, 6.02 × 1023 is known as Avogadro's constant.

One mole of hydrogen atoms does not have the same mass as one mole of uranium atoms. Hydrogen and uranium have different atomic masses. Moles are used to compare number, not mass.

Moles are very helpful when working with chemical reactions, as they can help us find out how much of a product will be produced in a certain reaction.

Finding amounts of atoms/molecules in substances

Now that we've covered moles, we can find the amount of particles in a substance.

Let's say we have 12 grams of Helium. Helium's atomic mass is 4.00 amu. How many moles of Helium do we have?

  • If 1 atom of Helium weighs 4.00 amu, then 1 mole of Helium weighs 4.00 grams.

Answer: We have 3 moles of Helium.

What if we have 90 grams of water? Roughly how many moles of H2O do we have?

  • Hydrogen's atomic mass is 1.00 amu.

  • Oxygen's atomic mass is 16.00 amu.

  • The mass of one molecule of H2O is therefore 18.00 amu.

  • 1 mole of H2O therefore weighs 18.00 grams.

Answer: We have 5 moles of H2O.

Practice

We have 256 grams of Oxygen molecules O~2~. How many moles of Oxygen molecules do we have? Enter your answer below as an integer: