Ch.3 - Chemical Reactions and Reaction Stoichiometry

Problem 45c

Chapter 3, Problem 45c

Give the empirical formula of each of the following compounds if a sample contains (c) 26.56% K, 35.41% Cr, and 38.03% O by mass.

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Calculate the moles of each element in the compound by dividing the percentage by the atomic mass of each element. Use the atomic masses: K (Potassium) = 39.1 g/mol, Cr (Chromium) = 52.0 g/mol, O (Oxygen) = 16.0 g/mol.

Convert the mass percentages to moles: Moles of K = 26.56% / 39.1 g/mol, Moles of Cr = 35.41% / 52.0 g/mol, Moles of O = 38.03% / 16.0 g/mol.

Determine the simplest whole number ratio of the moles of each element. Divide all the mole values by the smallest mole value obtained in the previous step.

Round the ratios to the nearest whole number if they are close to a whole number to get the empirical formula. This step might involve multiplying all ratios by a common factor to eliminate any decimals.

Write the empirical formula using the whole number ratios as subscripts for each element.

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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 of a compound represents the simplest whole-number ratio of the elements present in that compound. It is derived from the percentage composition of each element by mass, allowing chemists to understand the basic composition of the substance without detailing the actual number of atoms in a molecule.

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 the atoms in the empirical formula. Knowing the molar mass is essential for converting between mass and moles, which is crucial for determining the empirical formula from percentage compositions.

Percent Composition

Percent composition refers to the percentage by mass of each element in a compound. It is calculated by dividing the mass of each element in a sample by the total mass of the sample and multiplying by 100. This concept is fundamental for deriving the empirical formula, as it provides the necessary data to determine the ratio of elements in the compound.

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