Combustion analysis is performed on 0.50 g of a hydrocar-bon and 1.55 g of CO₂, and 0.697 g of H₂O are produced. The mass spectrum for the hydrocarbon is provided below. What is the molecular formula? (LO 3.12 and 3.13)
(a) C₅H₁₁ (b) C₈H₁₈ (c) C₁₁H₁₀ (d) C₁₀H₂₂

Verified step by step guidance

1. Step 1: Determine the mass of carbon and hydrogen in the hydrocarbon. This can be done by using the molar mass of CO2 and H2O and the fact that each molecule of CO2 contains one atom of carbon and each molecule of H2O contains two atoms of hydrogen.
2. Step 2: Subtract the mass of carbon and hydrogen from the total mass of the hydrocarbon to find the mass of any other elements present in the hydrocarbon.
3. Step 3: Convert the mass of each element to moles by dividing by the molar mass of that element.
4. Step 4: Divide the number of moles of each element by the smallest number of moles calculated in step 3. This will give the ratio of the elements in the hydrocarbon.
5. Step 5: Use the ratio of elements to determine the molecular formula of the hydrocarbon. The molecular formula is the actual number of atoms of each element in a molecule of the compound. Compare the ratio with the options given to find the correct molecular formula.

Key Concepts

Combustion Analysis

Combustion analysis is a technique used to determine the composition of a hydrocarbon by burning it in excess oxygen and measuring the amounts of carbon dioxide (CO2) and water (H2O) produced. The mass of CO2 produced indicates the amount of carbon in the hydrocarbon, while the mass of H2O indicates the amount of hydrogen. This data is crucial for calculating the empirical formula of the compound.

Empirical vs. Molecular Formula

The empirical formula represents the simplest whole-number ratio of atoms in a compound, while the molecular formula indicates the actual number of atoms of each element in a molecule. To find the molecular formula from the empirical formula, one must know the molar mass of the compound, which can be determined through mass spectrometry or other methods. Understanding the distinction between these formulas is essential for accurately identifying the compound.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on the conservation of mass. In combustion analysis, stoichiometric relationships allow for the conversion of the masses of CO2 and H2O produced into moles of carbon and hydrogen, respectively. This quantitative approach is fundamental for determining the empirical formula and ultimately the molecular formula of the hydrocarbon.