1. Percent Composition Percent Composition by Mass in a Compound

2. Aim: How can we determine the percentage composition by mass of elements in a compound? Essential Questions 1. What does “percent composition” mean? 2. How do we calculate the percent due to each element from the formula mass? 3. How do we calculate the percentage of water in a hydrate? Key Words: percentage, formula mass, component, element, compound, hydrate, anhydrous.

3. Do Now Find the formula mass of mercueric (II) chlorine

4. How Can We Calculate the Percent Composition of Each Element Compared to the Formula Mass? Define Percent Composition: Calculate Percent Composition:

5. Percent Composition Definition: The percent composition of a substance represents the composition as a percentage of each element compared with the total mass of the compound. Formula:

6. What are Hydrates? Ionic compounds often contain a fixed amount of water in their crystal structure. These molecules are indicated as part of the formula. Ex: CuSO 4  5H 2 O. Crystals which contain attached water molecules are called hydrates. Substances without water are known as anahydrous.

7. What is the Percent by Mass of Water in Na 2 CO 3  10H 2 O?

8. Calculating the Percentage of Water in a Hydrate When calculating the percentage of water in a hydrate, treat the entire water molecule as a single unit/component. Step 1: Calculate the formula mass of the crystal. Step 2: Calculate the percent of water in the compound.

9. Learning Check 1. What is the percent by mass of nitrogen in NH 4 NO 3 ?

10. Learning Check 2. What is the percent by mass of water in CaSO 4   2H 2 O?

11. Homework 1. Redo HW #3, exercises 1-5 using concepts learned in classroom today. 2. Calculate the percent by mass of water in Na 2 B 4 O  10H 2 O

12. (Notes) II. Percent Composition A. Anhydrous Substances - Substances without water trapped inside are known as anahydrous. 1. Definition: The percent composition of a substance represents the composition as a percentage of each element compared with the total mass of the compound. 2. Formula:

13. Example

14. Notes B. Hydrous Substances – Crystals which contain attached water molecules are called hydrates. 1. Ionic compounds often contain a fixed amount of water in their crystal structure. These molecules are indicated as part of the formula. Ex: CuSO 4  5H 2 O. 2. When calculating the percentage of water in a hydrate, treat the entire water molecule as a single unit/component. a. Step 1: Calculate the formula mass of the crystal. b. Step 2: Calculate the percent of water in the compound.

15. Example