1. PERCENT COMPOSITION X 100= % mass of element mass of element mass of compound

2. The relative amounts of the elements in a compound are expressed as the percent composition. The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100.

3. Propane(C 3 H 8 ), is the fuel commonly used in gas grills. Calculate the percent composition of propane. KNOWNS REMEMBER: part whole  Mass of C in propane  Mass of H in propane  Molar mass of propane CALCULATE % C = C__ propane = 36.0 g 44.0 g = 81.8% % H = H___ propane = 8.0 g 44.0 g = 18.2% X 100 PERCENT COMPOSITION FROM A FORMULA C: 12.0(3) = 36.0g/mol H: 1.0(8) = 8.0g/mol 44.0g/mol X 100

4. Try this with your partner... Find the percent composition of water. KNOWN  Formula of water  Mass of hydrogen in water  Mass of oxygen in water CALCULATE % H = 2.0 18.0 =X 100 11.1 % % O = 16.0 16.0 18.0 X 100 = 88.9 % PERCENT COMPOSITION FROM A FORMULA H2OH2O H: 1.0(2) = 2.0g/mol O: 16.0(1) = 16.0g/mol REMEMBER: part whole X 100

5. PERCENT COMPOSITION FROM MASS DATA When a 13.60 g sample of a compound containing only Mg & O is decomposed. 5.40 g of O is obtained. What is the % comp. of this cmp? KNOWN  Mass of compound = 13.60 g  Mass of oxygen = 5.40 g  Mass of magnesium when you subtract  13.60g – 5.40 g = 8.20 g Mg CALCULATE % Mg = 8.20 13.60 X 100 = 60.3 % % O = 5.40 13.60 X 100 = 39.7 % REMEMBER: part whole X 100 REMEMBER: If you are not given each of the elements SUBTRACT to find them!

6. PERCENT COMPOSITION FROM MASS DATA Try this with your partner... A 34.0 g sample of a compound containing 8.5 g of O is formed. The remaining portion is composed of Ca. What is the % composition of this compound? KNOWN 25% oxygen 75.0% calcium Mass of compound = 34.0 g Mass of oxygen = 8.5 g Mass of calcium when you subtract 34.0g – 8.5 g = 25.5 g Ca CALCULATE REMEMBER: part whole REMEMBER: If you are not given each of the elements SUBTRACT to find them! X 100