Chapter 7: Chemical Formulas and Chemical Compounds Section 3: Using Chemical Formulas

Overview We will calculate the formula mass or molar mass of any given substance. We will work on molar conversions for molecules. We will calculate the number of molecules or formula units in a given molar amount. We will calculate the percent composition of a chemical compound.

Formula Mass The sum of all average atomic masses (molar masses) of all the atoms represented in the chemical formula. Recall the average atomic mass of H = 1.01g, which is the same as the molar mass of H which equals 1.01g/mol. So what is the formula mass of H2O2? 2 H x 1.01 g = 2.02 g 2 O x 16.00 g = 32.00 g Total H + Total O = 34.02 g

Molar Mass It is found the same way. They are numerically equal to each other. So what is the molar mass of H2O2? 2 H x 1.01 g/mol = 2.02 g/mol 2 O x 16.00 g/mol = 32.00 g/mol Total H + Total O = 34.02 g/mol

Molar Conversions Recall the value of g/mol can be used to convert the number of moles of a substance to moles or grams. Also remember that to find the number of molecules in a substance you will need to how many moles of the substance and take that times Avogadro’s Number. 1 mol = 6.022x1023 = molar mass = 22.4 L (STP)

Percent Composition The percent by mass of each element in a compound. So what is the percent composition of H2O2? First find the formula mass. 2 H x 1.01 g = 2.02 g 2 O x 16.00 g = 32.00 g Total H + Total O = 34.02 g Then divide the total mass of each element by the formula mass and take it times 100. H = 5.94% O = 94.06%