Determine the empirical and molecular formulas of each of the following substances: (b) Caffeine, a stimulant found in coffee, contains 49.5% C, 5.15% H, 28.9% N, and 16.5% O by mass and has a molar mass of 195 g/mol.

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Convert the percentage composition to grams for each element assuming a 100 g sample. This means you have 49.5 g of Carbon (C), 5.15 g of Hydrogen (H), 28.9 g of Nitrogen (N), and 16.5 g of Oxygen (O).

Calculate the moles of each element by dividing the mass of each element by its atomic mass (C: 12.01 g/mol, H: 1.008 g/mol, N: 14.01 g/mol, O: 16.00 g/mol).

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

Use the mole ratio to write the empirical formula by assigning these ratios as subscripts to the respective elements.

Calculate the molecular formula by dividing the molar mass of the compound (195 g/mol) by the molar mass of the empirical formula. Multiply the subscripts in the empirical formula by this factor to get the molecular formula.

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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 mole ratios are simplified to the smallest integers. This formula provides insight into the basic composition of the substance without indicating the actual number of atoms in a molecule.

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 multiplying the subscripts by a whole number, which is determined by dividing the compound's molar mass by the molar mass of the empirical formula. This formula gives a complete picture of the molecular structure and composition.

Molar Mass

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 a molecular formula. Knowing the molar mass is essential for converting between grams and moles, which is crucial for determining both empirical and molecular formulas in stoichiometric calculations.

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