4.6 Molecular Formulas
3 Steps to Determining Chemical Formulas
1. Determine the percent composition of all elements.
2. Convert this information into an empirical formula
3. ______
Molecular Formulas
• Molecular Formula of a compound tells you ______of a compound. This formula may be equal to the empirical formula or may be a multiple of this formula.
• To determine, you need:
• ______
• ______
• Empirical Formula
• shows the ______
Example: ______
• Molecular Formula
• shows the ______
Example: ______
Steps to Determine Molecular formula:
1. List the given values (if the empirical formula is not given, you must determine that before moving on).
2. Determine the molar mass for the empirical formula.
3. Divide the Molecular molar mass by the empirical formula molar mass.
4. Calculate Molecular Formula by multiplying this number by the empirical formula.
Problems
1. The empirical formula of a compound is CH3O and its molar mass is 93.12g/mol. What is the molecular formula? [C3H9O3]
2. The percent composition of a compound is determined by a combustion and analyzer is a 40.03% carbon, 6.67% hydrogen, & 53.30% oxygen. The molar mass is 180.18g/mol. What is the molecular formula? [C6H12O6]
3. The percent composition of a compound is determined by a combustion analyzer is a 32.0% carbon, 6.70% hydrogen, 42.6% oxygen & 18.7% nitrogen. The molar mass is 75.08g/mol. What is the molecular formula? [C2H5O2N]
Molecular Formula Assignment
Determine the molecular formulas for the following compounds. Place the elements in the formula as you see them presented in the question:
1. A compound has the following percentage composition: 26.7% carbon; 2.2% hydrogen; 71.1% oxygen. The molecular weight of this compound is 90g. What is the compound’s true formula? [C2H2O4]
2. A certain compound was analyzed and found to have the following composition: 54.6% carbon; 9.0% hydrogen; 36.4% oxygen. The true molecular weight for this compound is 176g. What is the molecular formula of the compound? [C8H16O4]
3. The percentage composition of ethane gas is 80.0% carbon and 20.0% hydrogen. The molecular weight for ethane is 30g. What is the correct formula for this compound? [C2H6]
4. Analysis of a compound shows that it consists of 24.3% carbon, 4.1% hydrogen, and 71.6% chlorine. The molecular weight of the compound is determined to be 148.44g. What molecular formula corresponds to these data? [C3H6Cl3]
5. An unknown compound is analyzed and found to consist of 49.0% carbon, 2.7% hydrogen, and 48.2% chlorine. Boiling point data suggest that the molecular weight of the compound is about 150g. What molecular formula would you predict for this compound? [C6H4Cl2]
6. A gaseous compound is found to have the following composition; 30.5% nitrogen and 69.5% oxygen. The molecular weight of the gas is found to be 98.1g. What molecular formula corresponds to this data? [N2O4]
7. The empirical formula of a compound is NO2. Its molecular mass is 92g/mol. What is its molecular formula? [N2O4]
8. The empirical formula of a compound is CH2. Its molecular mass is 70g/mol. What is its molecular formula? [C5H10]
9. A compound is found to be 40.0% carbon, 6.7% hydrogen and 53.5% oxygen. Its molecular mass is 60.0g/mol. What is its molecular formula? [C2H4O2]
10. A compound is 64.9% carbon, 13.5% hydrogen and 21.6% oxygen. Its molecular mass is 74g/mol. What is its molecular formula? [C4H9O]
11. A compound is 54.5% carbon, 9.1% hydrogen and 36.4% oxygen. Its molecular mass is 88g/mol. What is its molecular formula? [C4H8O]
CHEMICAL QUANTITIES / CHEMISTRY 20