1. Chapter 3 Mass Relationships in Chemical Reactions
Chemistry: McMurry and Fay, 6th Edition
Chapter 3: Mass Relationships in Chemical Reactions
4/24/2019 5:38:42 AM
Chapter 3 Mass Relationships in Chemical Reactions
Copyright © 2011 Pearson Prentice Hall, Inc.

2. Percent Composition and Empirical Formulas
Chapter 3: Formulas, Equations, and Moles
4/24/2019
Percent Composition and Empirical Formulas
molecular mass
empirical formula mass
multiple =
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3. Steps in determine the Empirical formula
Example: Determine the empirical formula for an unknown sample AxBy
Step 1: Obtain the mass of each element (in grams)
Step 2: Determine the number moles of each atom present
Step 3: Divide the smallest moles by numbers of each atom to obtain the closet integer as possible.
Step 4: If the result ended with 0.5, 0.33, 1.125, 1.50 etc… then multiply with a factor to get the nearest integer as possible.

4. Example
A compound containing nitrogen and oxygen is decomposed in the laboratory and produces 24.5 g nitrogen and 70.0 g oxygen. Calculate the empirical formula of the compound.

5. Example
An unknown sample gives the following mass percent: 17.5% Na, 39.7% Cr and 42.8% O. What is the empirical formula?

6. Determining Empirical Formulas: Elemental Analysis
Chapter 3: Formulas, Equations, and Moles
Determining Empirical Formulas: Elemental Analysis
4/24/2019
Combustion Analysis: A compound of unknown composition (containing a combination of carbon, hydrogen, and possibly oxygen) is burned with oxygen to produce the volatile combustion products CO2 and H2O, which are separated and weighed by an automated instrument called a gas chromatograph.
hydrocarbon + O2(g)
xCO2(g) + yH2O(g)
carbon
hydrogen
If the unknown compound also contains oxygen, then the masses of carbon and hydrogen are determined first. The mass of oxygen is determined by subtracting those masses from that of the unknown compound.
Copyright © 2008 Pearson Prentice Hall, Inc.

7. Percent Composition and Empirical Formulas
Chapter 3: Formulas, Equations, and Moles
4/24/2019
Percent Composition and Empirical Formulas
Upon combustion, a compound containing only carbon and hydrogen produces 1.83 g CO2 and g H2O. Find the empirical formula of the compound
Copyright © 2008 Pearson Prentice Hall, Inc.

8. Example
Upon combustion, a g sample of a compound containing only carbon, hydrogen, and oxygen produced g CO2 and g H2O. Find the empirical formula of the compound

9. Example
A compound contains only carbon, hydrogen, and oxygen. Combustion of g of compound yields g CO2 and g H2O. Determine the molecular formula of this compound. The molar mass of the compound is g/mol.