Compounds and the Mole

·         If a substance has more than one element, add up the molar mass of the element.

·         For example: H₂O has two atoms of hydrogen and one atom of oxygen. You figure it out by adding the molar masses of the two different elements.

o   2H + O = molar mas of H₂O

o   2(1.01 g) + 16.00 g = 18.02 g/mol

·         These units are expressed in grams per mol because the molecular weight is per 1 mol of the substance.

·         Example Problem: Find the molar mass of Al₂(SO₄)₃.

o   First, we make our equation:

§  2Al + 3S + 12O = molar mass of Al₂(SO₄)₃

o   Second, we plug in our numbers:

§  2(26.98 g) + 3(32.07 g) + 12(16.00 g) = 342.2 g/mol