Molar mass - the mass of a single atom of an element (in amu) is numerically equal to the mass of one mole of those atoms (in grams).

From the Periodic Table, the mass of one atom of chromium is 51.9961 amu.
The mass of one mole (6.02 X 10²³) of chromium atoms is 51.9961 grams.

Coefficients in a chemical equation are normally used to indicate a ratio of reacting molecules. They also indicate a ratio of reacting moles.

The mole concept allows chemists to use chemical equations to determine the "weight" of reactants and products in chemical reactions.

For example:   2Mg_(s) + O_2(g) → 2MgO_(s)

The balanced equation above shows that the product is 2 molecules of magnesium oxide. That also means 2 moles of magnesium oxide. By calculating the molar mass of MgO, then multiplying by the coefficient 2, the mass of MgO produced can be predicted from the balanced equation.

• If 2 moles of Mg_(s) are used − 2 X 24 u = 48 g Mg
• If 1 mole of O_2(g) is used − 1 X 32 u = 32 g O₂
• Then 2 moles of MgO_(s) are produced − 2 X 40 u = 80 g MgO

Notice that the total mass of the reactants (80 grams)
is equal to the total mass of the product (80 grams)!!