# More chemical quantities - hcs.stier.org More chemical quantities Percent composition and empirical formulas Calculating Percent Composition of a Compound Like all percent problems: part x 100 % = percent whole

1) Find the mass of each of the components (the elements), 2) Next, divide by the total mass of the compound; then x 100 2 Example Calculate the percent composition of a compound that is made of 29.0 grams of Ag with 4.30 grams of S. 29.0 g Ag X 100 = 87.1 % Ag

33.3 g total 4.30 g S X 100 = 12.9 % S 33.3 g total Total = 100 % 3 Getting it from the formula If we know the formula, assume you have 1 mole, then you know the mass of the elements and the whole

compound (these values come from the periodic table!). 4 Examples Calculate the percent composition of C2H4 5 Formulas Empirical formula: the lowest whole number ratio of atoms in a compound. Molecular formula: the true number of

atoms of each element in the formula of a compound. Example: molecular formula for benzene is C6H6 (note that everything is divisible by 6) Therefore, the empirical formula = CH (the lowest whole number ratio) 6 Formulas (continued)

Formulas for ionic compounds are ALWAYS empirical (the lowest whole number ratio = cannot be reduced). Examples: NaCl MgCl2 Al2(SO4)3 K2CO3 7

Formulas (continued) Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio). Molecular: H2O C6H12O6 C12H22O11 H2O

CH2O C12H22O11 (Correct formula) Empirical: (Lowest whole number ratio) 8 Calculating Empirical

Just find the lowest whole number ratio C6H12O6 CH4N A formula is not just the ratio of atoms, it is also the ratio of moles. In 1 mole of CO2 there is 1 mole of carbon and 2 moles of oxygen. In one molecule of CO2 there is 1 atom of C and 2 atoms of O. 9 Empirical to molecular Since the empirical formula is the lowest ratio, the actual molecule would

weigh more. By a whole number multiple. Divide the actual molar mass by the empirical formula mass you get a whole number to increase each coefficient in the empirical formula Look at example in workbook 10