1. Percent Composition Parts of a Whole

2. Percent Short for per centum …”by the hundred” Part of a whole If there are 10 people in a room and 4 of them have blue eyes, what percentage of people have blue eyes?

3. Composition Chemical formula Molar mass

4. Percent Composition Write the formula Find the molar mass Determine the percentage of the part in question

5. Problem #1 Determine the percent composition of water.

6. Problem #1—Step 1 Write its formula ▫ H 2 O

7. Problem #1—Step 2 Determine its molar mass ▫ H—2 x 1.01 = 2.02 ▫ O—1 x 16.00 = 16.00 18.02 g/mol

8. Problem #1—Step 3 Determine the portion that each element makes up by its mass and multiply by 100…determine the percent composition ▫ H—2 x 1.01 = 2.02 x 100 = 11.2% H 18.02 ▫ O—1 x 16.00 = 16.00 x 100 = 88.8% O 18.02

9. Problem #2 Determine the percent composition of aluminum carbonate.

10. Problem #2—Step 1 Write its formula ▫ Al 2 (CO 3 ) 3

11. Problem #2—Step 2 Determine its molar mass ▫ Al—2 x 26.98 = 53.96 ▫ C—3 x 12.01 = 36.03 ▫ O—9 x 16.00 = 144.00 233.99 g/mol

12. Problem #1—Step 3 Determine the percent composition ▫ Al—2 x 26.98 = 53.96 x 100 = 23.1% Al 233.99 ▫ C—3 x 12.01 = 36.03 x 100 =15.4% C 233.99 ▫ O—9 x 16.00 = 144.00 x 100 =61.5% O 233.99

13. Problem #3 A compound contains only copper and chloride ions. It is found to be 47.3% copper. Determine the formula of the compound.

14. Problem #3—Step 1 Determine the possible formulas ▫ Copper can have charges of 1+ or 2+ ▫ So, your choices are: CuCl and CuCl 2

15. Problem #3—Step 2 Determine both molar masses ▫ Cu—1 x 63.55 = 63.55 ▫ Cl—1 x 35.45 = 35.45 99.00 g/mol ▫ Cu—1 x 63.55 = 63.55 ▫ Cl—2 x 35.45 = 70.90 134.45 g/mol

16. Problem #3—Step 3 Determine the percentage of copper in each. ▫ Cu—1 x 63.55 = 63.55 x 100 = 64.2% Cu 99.00 ▫ Cu—1 x 63.55 = 63.55 x 100 = 47.3% Cu 134.45 So, the answer is CuCl 2

17. Problem #4 A compound is known to contain 41.7% magnesium. Determine if the compound is magnesium fluoride, magnesium hydroxide, or magnesium oxide.

18. Problem #4—Step 1 Determine the formulas ▫ MgF 2 ▫ Mg(OH) 2 ▫ MgO

19. Problem #4—Step 2 Determine all three molar masses ▫ Mg—1 x 24.31 = 24.31 ▫ F—2 x 19.00 = 38.00 62.31 g/mol ▫ Mg—1 x 24.31 = 24.31 ▫ O—2 x 16.00 = 32.00 ▫ H—2 x 1.01 = 2.02 58.33 g/mol ▫ Mg—1 x 24.31 = 24.31 ▫ O—1 x 16.00 = 16.00 40.31 g/mol

20. Problem #4—Step 4 Determine the percentage of magnesium in each. ▫ Mg—1 x 24.31 = 24.31 x 100 =39.0% Mg 62.31 ▫ Mg—1 x 24.31 = 24.31 x 100 = 41.7% Mg 58.33 ▫ Mg—1 x 24.31 = 24.31 x 100 =60.3% Mg 40.31

21. Problem #4—Answer Mg(OH) 2

22. Practice, practice, practice