1. Chapter 11 Notes, part II -% composition

2. In Review The mole is an SI unit of measureThe mole is an SI unit of measure for amount of particles for amount of particles 6.02x10 23 particles = 1 mole6.02x10 23 particles = 1 mole Molar mass=1 moleMolar mass=1 mole

3. % Composition The percent composition of an element is the relative amount of that element in a compound.The percent composition of an element is the relative amount of that element in a compound. You find the percent composition of an element by dividing the element’s mass by the mass of the entire compound.You find the percent composition of an element by dividing the element’s mass by the mass of the entire compound. mass of element mass of element mass of compound x 100

4. For example: What is the % composition of carbon dioxide?What is the % composition of carbon dioxide?

5. Mass of oxygen = 2(16.0)=32.0 g Mass of carbon = 12.0 g Mass of carbon dioxide = 44.0 g 12.0 g C 12.0 g C 44.0 g CO 2 44.0 g CO 2 32.0 g O 32.0 g O 44.0 g CO 2 For example: x100 = 27.3% Carbon x100 = 72.7% Oxygen

6. Try these… What is the percent of each element in phosphorus trichloride?What is the percent of each element in phosphorus trichloride? If 20.0 grams Ca react completely with 16.0 grams of S to form a compound, what is the percent of S?If 20.0 grams Ca react completely with 16.0 grams of S to form a compound, what is the percent of S?

7. Empirical Formula The empirical formula gives you the lowest, whole number ratio of elements in the compound.The empirical formula gives you the lowest, whole number ratio of elements in the compound. The empirical formula may or may not be the same as the molecular formula.The empirical formula may or may not be the same as the molecular formula.

8. For example: For carbon dioxide, the molecular formula is CO2, and the empirical formula is CO2. One carbon and two oxygens are the lowest ratio of atoms.For carbon dioxide, the molecular formula is CO2, and the empirical formula is CO2. One carbon and two oxygens are the lowest ratio of atoms. The molecular formula for dinitrogen tetrahydride is N2H4, but the empirical formula is NH2.The molecular formula for dinitrogen tetrahydride is N2H4, but the empirical formula is NH2.

9. What is the empirical formula for: C 6 H 12 O 6C 6 H 12 O 6 C 6 H 12 O 2C 6 H 12 O 2 N 2 H 2N 2 H 2 CH 4CH 4

10. To Find the Empirical Formula from % Composition: If given the percentages, assume there are 100.0 grams of the compound.If given the percentages, assume there are 100.0 grams of the compound. Convert the grams of each element to moles.Convert the grams of each element to moles. Divide by the smaller amount of moles, then manipulate the ratio so that all numbers are whole.Divide by the smaller amount of moles, then manipulate the ratio so that all numbers are whole.

11. Two Examples: What is the empirical formula of a compound that is 27.3% carbon and 72.7% oxygen?What is the empirical formula of a compound that is 27.3% carbon and 72.7% oxygen? What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?

12. More to try… Calculate the empirical formula of a compound that is 94.1% oxygen, 5.9% hydrogen.Calculate the empirical formula of a compound that is 94.1% oxygen, 5.9% hydrogen. Calculate the empirical formula of a compound that is 79.8% carbon, 20.2% hydrogen.Calculate the empirical formula of a compound that is 79.8% carbon, 20.2% hydrogen.

13. From there, find it’s molecular formula: If given the molar mass (how many g/mol of the compound) then you can calculate the molecular formula from the empirical formula.If given the molar mass (how many g/mol of the compound) then you can calculate the molecular formula from the empirical formula. Take the compound’s empirical formula mass and compare to the molecular mass. The molecular mass will be a multiple of the empirical formula’s mass.Take the compound’s empirical formula mass and compare to the molecular mass. The molecular mass will be a multiple of the empirical formula’s mass.

14. Example: If the molecular mass of the first example (N2O5) problem is 216 g/mol, then what is the molecular formula for the compound?If the molecular mass of the first example (N2O5) problem is 216 g/mol, then what is the molecular formula for the compound?

15. Try these: What is the molecular formula of a compound whose molar mass is 60.0 g and whose empirical formula is CH4N?What is the molecular formula of a compound whose molar mass is 60.0 g and whose empirical formula is CH4N? What is the molecular formula for a compound whose molar mass is 78 g and whose empirical formula is CH?What is the molecular formula for a compound whose molar mass is 78 g and whose empirical formula is CH?