1. Molar Mass Practice: Molar Mass Practice: 1. Find the molar mass of ammonium sulfate (also called the formula mass): 2. Find the molar mass of copper (II) chloride: Percent Composition, Empirical, and Molecular Formula Problems

2. Percent Composition by Mass – the percent, by mass, of each element in a compound. Percent Composition by Mass – the percent, by mass, of each element in a compound. If you have a box containing 100 golf balls and 100 ping pong balls, which type of ball contributes the most to the mass of the box? If you have a box containing 100 golf balls and 100 ping pong balls, which type of ball contributes the most to the mass of the box? The same principle applies to finding the % composition of a compound. Different elements have different masses and this must be taken into consideration. The same principle applies to finding the % composition of a compound. Different elements have different masses and this must be taken into consideration.

3. % = part X 100 whole What percent of air is oxygen? If 200. grams of air contain 42 grams of oxygen what percent of air is oxygen? % = 42g X 100 = 21% 200g 200g

4. % = part X 100 whole What percent of air is oxygen?

5. How to find the percent composition of a compound: 1. Write a correct formula for the compound 2. Find the molar mass of the compound 3. Divide the total atomic mass of EACH ELEMENT by the molar mass 4. Multiply by 100 to convert your results to a percent 5. Express your answer to TWO decimal places with the % sign.

6. Practice: 1. Find the percent composition by mass of zinc carbonate. 2. Find the percentage composition by mass of aluminum nitrate.

7. Empirical and Molecular Formulas Empirical Formulas - are formulas which have been reduced to their lowest terms (we write all formulas for ionic compounds as empirical formulas). Empirical Formulas - are formulas which have been reduced to their lowest terms (we write all formulas for ionic compounds as empirical formulas). Empirical formulas can be calculated from 2 types of data: a. you may be given the percentage of each element present in the compound b. you may be given the actual number of grams of each element present in the compound

8. Here is a way to remember the steps : percent to mass, mass to mole, divide by small, multiply till whole Example: Find the empirical formula for a compound which contains 26.8% Sn, 16.0% Cl and 57.2% I Example: 100. grams of a compound contains 22.430 grams of carbon, 6.542 grams of hydrogen, 44.8598 grams of oxygen and 26.1682 grams of nitrogen. Find the empirical formula.

9. Molecular Formulas To determine the molecular formula, you must know the compound’s empirical formula AND the molar mass of the molecular compound. Example: C 2 H – empirical formula C 12 H 6 – molecular formula C 12 H 6 – molecular formula

10. How to find the molecular formula: 1. Calculate the mass of the empirical formula (which you have already found or it will be given to you ) 2. Divide the known molar mass by the mass of the empirical formula. 3. Multiply that number by the subscripts of the empirical formula to get the subscripts for the molecular formula.

11. Example: The molar mass of a compound is 181.50 g/mol and the empirical formula is C 2 HCl. What is the molecular formula? Example: Find the empirical formula for a compound containing only carbon and hydrogen if it is known to contain 84.21% carbon.

12. b. If the molar mass is 114 g/mol, what is the molecular formula of this compound?