PERCENT COMPOSITION CHAPTER 7-3

Percent Composition The relative amounts of each element in a compound are expressed as percent composition or by the percent composition by mass of each element in a compound. To calculate the percent composition of a compound, we must use the following formula: Molar mass element % Composition = x 100 Molar mass compound Percent composition Percent mass How to calculate percent composition

Sample Problem Possible Test Question (PTQ) Calculate the percent composition of CH3OH. Find the molar mass of the compound. C = 1 x 12 = 12 H = 4 x 1 = 4 O = 1 x 16 = 16 32 g/mol

Use % composition formula. 12 g/mol % C = x 100 = 37.5% C 32 g/mol 4 g/mol % H = x 100 = 12.5% H 16 g/mol % O = x 100 = 50% O

Percent Composition by Mass How to calculate the percent mass of an element in a compound? The percent by mass of an element in a compound is the number of grams of the element divide by the number of grams of the compound, multiplied by 100. grams of element % mass = x 100 grams of compound

Sample Problem Possible Test Question (PTQ) Law of Conservation of mass An 8.20 piece of magnesium combines completely with 5.40 g of oxygen to form a compound. What is the percent composition of this compound? Find the mass of the compound. 2 Mg + O2  2 MgO 8.20 g + 5.40 g = 13.6 g

Use % mass formula 8.20 g Mg % Mg = x 100 = 60.3 % Mg 13.6 g MgO 5.40 g O % O = x 100 = 39.7 % O