Ch.3 - Chemical Reactions and Reaction Stoichiometry

Problem 93

Chapter 3, Problem 93

Cinnamaldehyde is a compound that is responsible for the characteristic aroma of cinnamon. It contains 81.79% C, 6.10% H, and the remaining is oxygen. Its molar mass is 132 g/mol. Determine its molecular formula.

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Calculate the mass percentage of oxygen by subtracting the percentages of carbon and hydrogen from 100%.

Convert the mass percentages of C, H, and O to grams, assuming a 100 g sample of cinnamaldehyde.

Convert the grams of each element to moles by dividing by their respective atomic masses: C (12.01 g/mol), H (1.008 g/mol), and O (16.00 g/mol).

Determine the simplest whole number ratio of moles of each element by dividing each by the smallest number of moles calculated.

Use the molar mass of cinnamaldehyde (132 g/mol) to find the molecular formula by comparing the empirical formula mass to the molar mass and multiplying the subscripts in the empirical formula accordingly.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Empirical Formula

The empirical formula represents the simplest whole-number ratio of the elements in a compound. To determine it, the percentage composition of each element is converted to moles, and then the ratio of these moles is simplified. This formula provides a foundational understanding of the compound's composition, which is essential for deriving the molecular formula.

Molecular Formula

The molecular formula indicates the actual number of atoms of each element in a molecule of a compound. It can be derived from the empirical formula by determining the molar mass of the empirical formula and comparing it to the compound's molar mass. This relationship helps in identifying the true structure of the compound, which is crucial for understanding its properties.

Molar Mass Calculation

Molar mass is the mass of one mole of a substance, typically expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in the molecular formula. Knowing the molar mass is vital for converting between grams and moles, and it plays a key role in determining the molecular formula from the empirical formula.

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