1. Warm Up
Chemical analysis determines that a substance contains 21.6% sodium, 33.3% chlorine, and 45.1% oxygen. Determine the empirical formula of the substance.

2. Molecular Formulas
Recap difference between empirical and molecular formulas
LG: Given an empirical formula and mass spectroscopy data, I can determine the molecular formula of a substance

3. Minds On… CH2O or C2H4O2 or C3H6O3 or C6H12O6 ????
Given that the empirical formula of a compound is CH2O, what other piece of information would need to determine the molecular formula?
CH2O or C2H4O2 or C3H6O3 or C6H12O6 ????

4. Mass Spectroscopy
The largest fragment determines the molar mass of the compound
Chemists use mass spectroscopy data and empirical formulas to determine a molecular formula
During mass spectroscopy, the substance being tested is ionized. This causes most of the sample to fragment (or break into smaller pieces). It is assumed that some of the sample remains intact, so the largest fragment is the molar mass of the compound.
When we refer to the chemical formula for a molecular compound we mean the molecular formula. The chemical formula for an ionic compound is an empirical formula.

5. How were you able to determine the answer?
Try This…
A substance is found to have the empirical formula C2H4O and a molar mass of g/mol. What is the molecular formula of this compound?
How were you able to determine the answer?

6. Example 1
Fire investigators are trying to determine the cause of a forest fire.
Chemical analysis of the accelerant determines that the fuel is 82.5% carbon and 17.5% hydrogen.
Mass spectroscopy determines that the molar mass of the substance is 58.0 g/mol. What substance was used to start the fire?

7. Example 2
A sample of an unknown white powder is found at the scene of a crime. Combustion analysis determines that the substance contains 0.045g of hydrogen, 0.546g of oxygen and 0.409g of carbon
Mass spectroscopy determines that the molar mass of the substance is g/mol. Determine the chemical formula of the unknown substance
Discuss combustion analysis – can be used for hydrocarbons. Substance is burned in oxygen. The amount of carbon dioxide and water produced is used to calculate the mass of carbon, hydrogen and oxygen in the original compound.
Vitamin C (ascorbic acid) C6H8O6

8. Homework Old Text: Pg. 193 # 8, 9, 10 Pg. 197 # 6
New text: Pg. 300 #2, 6, 7, 8