Molecular formulas Transitional Reasoning Practice

Questions

1. 1.

   An unknown organic compound is found by elemental analysis to consist of 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. In a separate experiment, the complete combustion of a 0.150 mol sample of this compound produces 19.8 g of carbon dioxide ($\text{CO}_2$). What is the molecular formula of this organic compound? (Atomic masses: $\text{C} = 12.0\text{ g/mol}$, $\text{H} = 1.0\text{ g/mol}$, $\text{O} = 16.0\text{ g/mol}$)

   • A) $\text{C}_2\text{H}_4\text{O}_2$
   • B) $\text{C}_3\text{H}_6\text{O}_3$
   • C) $\text{C}_4\text{H}_8\text{O}_4$
   • D) $\text{C}_6\text{H}_{12}\text{O}_6$
2. 2.

   Under identical conditions of temperature and pressure, a sealed 2.00 L flask contains 2.40 g of an unknown gaseous hydrocarbon. When the flask is completely evacuated and filled with oxygen gas ($\text{O}_2$) under the same temperature and pressure, the oxygen gas has a mass of 2.56 g. If elemental analysis indicates that the hydrocarbon consists of 80.0% carbon and 20.0% hydrogen by mass, what is the molecular formula of the compound? (Atomic masses: $\text{C} = 12.0\text{ g/mol}$, $\text{H} = 1.0\text{ g/mol}$, $\text{O} = 16.0\text{ g/mol}$)

   • A) $\text{CH}_3$
   • B) $\text{C}_2\text{H}_4$
   • C) $\text{C}_2\text{H}_6$
   • D) $\text{C}_3\text{H}_8$
3. 3.

   A materials chemist analyzes a volatile oxide of nitrogen utilized as an oxidizer in rocket propulsion systems. A pure gaseous sample containing $3.01 \times 10^{22}$ molecules is found to have a mass of 4.60 g. If nitrogen accounts for 30.4% of the compound's total mass, what is the molecular formula of this substance? (Avogadro's number = $6.02 \times 10^{23}\text{ mol}^{-1}$; atomic masses: $\text{N} = 14.0\text{ g/mol}$, $\text{O} = 16.0\text{ g/mol}$)

   • A) $\text{N}_2\text{O}_4$
   • B) $\text{NO}_2$
   • C) $\text{N}_2\text{O}_3$
   • D) $\text{N}_2\text{O}_5$
4. 4.

   An analytical chemist isolates a fragrant liquid containing only carbon, hydrogen, and oxygen, determining its empirical formula to be $\text{C}_2\text{H}_4\text{O}$. To determine its molecular formula, a 0.880 g sample of the compound is completely vaporized, occupying 224 mL at standard temperature and pressure (STP). What is the molecular formula of this compound? (Atomic masses: $\text{C} = 12.0\text{ g/mol}$, $\text{H} = 1.0\text{ g/mol}$, $\text{O} = 16.0\text{ g/mol}$; molar volume of a gas at STP = $22.4\text{ L/mol}$)

   • A) $\text{C}_2\text{H}_4\text{O}$
   • B) $\text{C}_3\text{H}_6\text{O}_2$
   • C) $\text{C}_4\text{H}_8\text{O}_2$
   • D) $\text{C}_6\text{H}_{12}\text{O}_3$
5. 5.

   During an investigation of an unknown hydrocarbon fuel, elemental analysis reveals that it contains 85.6% carbon and 14.4% hydrogen by mass. In a companion measurement, a 0.250 mol sample of this gaseous compound is found to have a mass of 14.0 g. Based on these experimental results, what is the molecular formula of the fuel? (Atomic masses: $\text{C} = 12.0\text{ g/mol}$, $\text{H} = 1.0\text{ g/mol}$)

   • A) $\text{C}_4\text{H}_8$
   • B) $\text{CH}_2$
   • C) $\text{C}_2\text{H}_4$
   • D) $\text{C}_3\text{H}_6$

Answers and step-by-step solutions appear after you finish the test.