1. Back side of blue notes Top left section Percent composition by mass: the percentage of an element in a compound How: Find molar mass first (mass of element in a sample of a compound) / (molar mass) x 100% When: you see percent composition, mass percentage, percentage composition, BOTH % and mass in a question

2. Back side of blue notes Bottom left section EX: Find the percentage composition of copper (I) sulfide. Find formula: Cu + and S 2- combine to make Cu 2 S Find molar mass: Cu: 63.55g/mol x 2 ; S: 32.07g/mol x 1; total molar mass = 159.2 g/mol Percent of copper: 127.1 g Cu / 159.2 g Cu 2 S x 100% = 79.84% Cu Percent of sulfur: 32.07 g S / 159.2 g Cu 2 S x 100% = 20.14%

3. Back side of blue notes Top right section Empirical formula: the formula for a compound with the smallest whole number ratio of elements (most simplified) How: Start with mass ratio: if given mass of sample and elements, use mass of each element; if given % composition, assume 100g of sample and % = grams Use molar mass to convert from grams of element to moles Divide all mole answers by the smallest number of moles found If all numbers are close to whole numbers, round to nearest whole number; if answers are close to _.5, multiply ALL answers by 2 These numbers are the subscripts for your compound’s empirical formula

4. Back side of blue notes Bottom right section Leave blank: we will do this example in class