Moles and molar mass
Learn how to count atoms and molecules using Avogadro's number and the mole. Use the periodic table to calculate the molar mass of a substance.
▶ Average atomic mass
The average atomic mass (sometimes called atomic weight) of an element is the weighted average mass of the atoms in a naturally occurring sample of the element. Average masses are generally expressed in unified atomic mass units (u), where 1 u is equal to exactly one-twelfth the mass of a neutral atom of carbon-12.
▶ The mole and Avogadro's number
One mole of a substance is equal to 6.022 × 10²³ units of that substance (such as atoms, molecules, or ions). The number 6.022 × 10²³ is known as Avogadro's number or Avogadro's constant. The concept of the mole can be used to convert between mass and number of particles.
▶ Worked example: Calculating molar mass and number of moles
The molar mass of a substance is the mass in grams of 1 mole of the substance. As shown in this video, we can obtain a substance's molar mass by summing the molar masses of its component atoms. We can then use the calculated molar mass to convert between mass and number of moles of the substance.
Lessons after number 3 are reserved for Pro members — unlock the full course and every other one.