Avogadro’s Number

Avogadro’s Number is approximately 6.022 x 10²³. This number is extremely important in chemistry.

Mole: A mole of something is 6.022 x 10²³ of that thing. You could theoretically have a mole of anything. A mole of baseballs is 6.022 x 10²³ baseballs, a mole of hotdogs is 6.022 x 10²³ hotdogs, etc. However, in chemistry we usually use moles to describe numbers of particles. A mole of O₂ is 6.022 x 10²³ molecules of O₂. Because there are so many particles in the world, we use a large number like Avogadro’s Number to describe them.

What is Molar Mass

Molar mass has the units grams per mole (g/mol). It is used to describe the weight of one mole of a substance. For example, O₂ has a molar mass of about 32 g/mol. This means if I took 6.022 x 10²³ O₂ molecules, it would weigh 32 grams.

Atomic Mass Unit

The atomic mass unit (amu) is used to measure the weight of atoms or molecules. It is an extremely small measurement. In fact, 1 gram = 6.022 x 10²³ amu. Since amu = gram/mol, the amount of amu that a single atom/molecule weighs is also the molar mass of that substance.

amu = g/mol, and on most periodic tables you are given amu values for each element. You can use these values to calculate molar mass.

Example Problem

Find the molar mass of CO₂

Answer

C = 12.01 amu

O = 16.00 amu

CO₂ = C + 2(O) = 12.01 + 2(16.00) = 44.01 amu
Since amu = g/mol, the molar mass of CO₂ is 44.01 g/mol.

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