1. Percent Composition This tutorial will help you learn how to calculate the percentage by mass of elements in a compound

2. What is the percentage of oxygen in Potassium chlorate? Place 1.29g of KClO 3 in a crucible and heat it for at least 10 minutes. Heating this compound will drive the oxygen atoms out of the compound and into the air as oxygen gas. After heating, allow the crucible to cool to room temperature.

3. The mass of the contents in the crucible is now 0.78g. All of the oxygen has been driven off and only KCl remains in the crucible. What is the mass of oxygen that was driven off?

4. CORRECT 1.29g – 0.78g = 0.51g of oxygen Now determine the percentage of oxygen in the compound by taking the mass of they oxygen divided by the mass of the compound and multiplying by 100. Show your work on your paper.

5. GOOD WORK (0.51g/1.29g)x 100 = 39.1% oxygen Let’s now do some problems when we only know the formula of the compound.

6. What is the mass of carbon in carbon dioxide? #1 – Find the mass of the atom. Carbon has a mass of 12.0g #2 – Find the mass of the compound CO 2 has a molecular mass of 44.0g. CO 2 has a molecular mass of 44.0g. #3 – Mass of atom/mass compound and then multiply by 100. (12.0/44.0) x 100 = 27.3% carbon (12.0/44.0) x 100 = 27.3% carbon

7. Practice problems What is the percentage by mass of iron in Iron(III) chloride?

8. Iron(III) chloride is FeCl 3. Iron has a mass of 55.8g FeCl 3 has a molecular mass of 162.3 (55.8/162.3) x 100 = 34.4%

9. Practice problems What is the percentage by mass of oxygen in carbon monoxide?

10. Carbon monoxide is CO. Oxygen has a mass of 16.0g. CO has a molecular mass of 28.0g (16.0/28.0) x 100 = 57.1%

11. Practice problems What is the percentage by mass of chlorine in magnesium chloride?

12. Magnesium chloride is MgCl 2. The mass of chlorine is 35.5g, so two chlorines will have a mass of 71.0g. The mass of MgCl 2 is 95.3g. (71.0/95.3) x 100 = 74.5%