1. Empirical, molecular, percent composition

2. Percent Composition Percent by mass of each element in a compound
%mass of element = mass element/mass compound x 100%

3. Percent from mass data
When a g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the percent composition?
Total sample: = 19.0 g
Percent Mg: 13.60/19.0 =71.6%
Percent O: 5.40/19.0=28.4%
Double check: = 100! Got it!

4. Percent from formula
Propane (C3H8), the fuel commonly used in gas grills, is one of the compounds obtained from petroleum. Calculate the percent composition of propane.
Mass carbon: 3(12.01) = g/mol
Mass hydrogen: 8(1.01) = 8.08 g/mol
Total molar mass: =44.11 g/mol
Percent carbon: 36.03/44.11 = 81.7%
Percent hydrogen: 8.08/44.11 = 18.3%
Double check: = 100!! Got it!

5. Empirical Formula Lowest whole number ratio of atoms in a compound
May or may not be the same as the molecular formula
Can be experimentally determined
Example: HO is the lowest ratio of H to O

6. Determining empirical formula
A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound?
Percent to grams: 25.9% is 25.9 g, 74.1% is 74.1 g
Grams to moles
Nitrogen: 25.9/14.01 = 1.85 mol
Oxygen: 74.1/16 = 4.63 mol
Divide by least, then multiply to whole numbers
Nitrogen: 1.85/1.85 = 1 x 2 = 2
Oxygen: 4.63/1.85 = 2.5 x 2 = 5
Use as subscripts: N2O5

7. Molecular Formula
Same as empirical formula or a simple whole-number multiple of the empirical formula
Use the molar mass to determine
Example: HO can be paired as H2O2. Twice the empirical formula

8. Finding the molecular formula
Calculate the molecular formula of a compound whose molar mass is 60.0 g/mol and empirical formula is CH4N.
Calculate molar mass of empirical formula: = g/mol
Determine the multiplication factor: 60.0/30 = 2
Molecular formula is 2 times empirical
CH4N x 2 = C2H8N2

9. Comparison Formula Classification Molar mass CH Empirical 13
C2H2 (ethyne)
Molecular
26 (2 x 13)
C6H6 (benzene)
78 (6 x 13)
CH2O (methanal)
Empirical and molecular 30
C2H4O2 (ethanoic acid)
60 (2 x 30)
C6H12O6 (glucose)
180 (6 x 30)

10. More
Let Mr. Bozeman take it away!

11. Now you try:
1. A compound is formed when 9.03 g Mg combines completely with 3.48 g N. What is the percent composition of this compound?
2. Calculate the percent composition of these compounds:
A. ethane (C2H6)
B. sodium hydrogen sulfate (NaHSO4).
3. Calculate the empirical formula of each compound:
A. 94.1% O, 5.9% H
B. 67.6% Hg, 10.8% S, 21.6% O
4. Find the molecular formula of ethylene glycol, which is used as antifreeze. The molar mass is 62 g/mol and the empirical formula is CH3O.