example, one mole of an atom with atomic mass x amu has a mass of x grams. The same
relationship holds for molecules: One mole of a compound has a mass in grams equal to the
molecular weight of that compound in amu, and contains 6.022 × 10^23 molecules of the compound.
For example, the molecular weight of carbonic acid, H 2 CO 3 , is (2 × 1 + 12 + 3 × 16) = 62 amu. 62 g of
H 2 CO 3 represents one mole of carbonic acid and contains 6.022 × 10^23 H 2 CO 3 molecules. In other
words, the molar mass of H 2 CO 3 is 62 g/mol. This can also be arrived at by simply adding the molar
atomic mass of the atoms in the compound: 1 mole of H 2 CO 3 contains 2 moles of H atoms, 1 mole of
C atoms, and 3 moles of O atoms.
Given the weight of a sample, one can determine the number of moles present with the following
formula:
number of moles = weight of sample (g) ÷ molar mass (g/mol)Example: How many moles are in 9.52 g of MgCl 2?
Solution: First, find the molar mass of MgCl 2.1(24.31 g/mol) + 2(35.45 g/mol) = 95.21 g/molNow, solve for the number of moles.