1. Review Molar relationship and Molar conversion problems

2. Agenda: 1. Review molar relationships 2. Review molar conversion problems 3. Intro to Percent Composition 4. Guided Practice 5. Independent Practice Percent Composition and Chemical Formulas

3. Objective: My words: SWBAT use percent composition of compounds to formulate empirical and molecular formulas of compounds as demonstrated through a handout/worksheet. Your Words:…

4. Why learn this… Many times scientists are faced with unknown compounds in the lab. There is a white powder…what is it? Can you think of a time when an unknown compound was the cause of mass panic? Experimentally determining the chemical formula so we know what the compound is can actually save lives!

5. Connecting to you… Is your shirt made of 100% cotton or wool? Why or why not? Usually a shirt’s composition is a bunch of different types of materials… Also, if you have experience with lawn care, you may know that percents of different elements in the mixtures of fertilizers is important in different seasons. For example, you may use nitrogen rich fertilizer in the spring to green the grass and a fertilizer with a higher percentage of potassium in the fall to strengthen the root systems.

6. Percent Composition The relative amounts of elements in a compound can be expressed as the percent composition (or percent by mass) of each element in the compound.

7. Percent Composition The percent by mass of an element in a compound is the number of grams of the element divided by the mass (g) of the compound. Then multiplied by 100%.

8. Sample Problem 1 When a 13.6 g sample of a compound containing only magnesium and oxygen is decomposed, 5.4 g of oxygen is obtained. What is the percent composition of this compound?

9. Sample Problem 2 A compound is formed when 9.03 g of Mg combines with 3.48 g of N. What is the percent composition of this compound?

10. Sample Problem 3 When a 14.2 g sample of Mercury (II) Oxide is decomposed into its elements by heating, 13.2 g of mercury are obtained. What is the percent composition of the compound?

11. Percent composition from the chemical formula Let’s say you are not trying to find the formula but rather, have the formula in front of you! You can find the percent composition by mass of the compound by using the formula and the molar mass!

12. Sample Problem 4 Propane (C3H8), the fuel commonly used in gas grills, is one of the compounds obtained from petroleum. Calculate the percent composition of propane.

13. Sample Problem 5 Calculate the Percent composition of baking soda: Sodium hydrogen carbonate (sodium bicarbonate).

14. Sample Problem 6 Calculate the percent of nitrogen in the following common fertilizers: a. NH 3 b. NH 4 NO 3

15. Recap How do you calculate the percent by mass of an element in a compound when you know the exact masses but do not know the formula? How do you calculate the percent by mass of an element in a compound when you know the formula? What is the percent composition of CO 2 ?