Ch.3 - Molecules, Compounds & Chemical Equations

Problem 91

Chapter 3, Problem 91

The elemental mass percent composition of ibuprofen (a nonsteroidal anti-inflammatory drug [NSAID]) is 75.69% C, 8.80% H, and 15.51% O. Determine the empirical formula of ibuprofen.

Verified step by step guidance

Convert the given mass percentages of each element into grams assuming a 100 g sample. This means you have 75.69 g of C, 8.80 g of H, and 15.51 g of O.

Calculate the number of moles of each element by dividing the mass of each element by its atomic mass: use 12.01 g/mol for C, 1.008 g/mol for H, and 16.00 g/mol for O. Express this as

n = \frac{mass}{atomic mass}

for each element.

Determine the mole ratio of the elements by dividing each element's mole value by the smallest number of moles calculated among the elements.

If the mole ratios are not whole numbers, multiply all ratios by the smallest factor that converts them into whole numbers (e.g., 2, 3, 4, etc.). This will give the subscripts for each element in the empirical formula.

Write the empirical formula by assigning the whole number mole ratios as subscripts to the respective elements C, H, and O.

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Key Concepts

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

Mass Percent Composition

Mass percent composition expresses the percentage by mass of each element in a compound. It is used to determine the relative amounts of elements present, which is essential for calculating the empirical formula by converting these percentages into moles.

Mole Concept and Conversion

The mole concept allows conversion from mass to moles using atomic masses. By dividing the mass percent of each element by its atomic mass, you find the mole ratio of elements, which is crucial for determining the simplest whole-number ratio in the empirical formula.

Empirical Formula Determination

The empirical formula represents the simplest whole-number ratio of atoms in a compound. After calculating moles of each element, divide all mole values by the smallest mole number to find the ratio, then round to the nearest whole number to write the empirical formula.

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Related Practice

Textbook Question

Calculate the empirical formula for each stimulant based on its elemental mass percent composition.

a. nicotine (found in tobacco leaves): C 74.03%, H 8.70%, N 17.27%

b. caffeine (found in coffee beans): C 49.48%, H 5.19%, N 28.85%, O 16.48%

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Textbook Question

Calculate the empirical formula for each natural flavor based on its elemental mass percent composition. a. methyl butyrate (component of apple taste and smell): C 58.80%, H 9.87%, O 31.33%

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Textbook Question

Calculate the empirical formula for each natural flavor based on its elemental mass percent composition. b. vanillin (responsible for the taste and smell of vanilla): C 63.15%, H 5.30%, O 31.55%

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Textbook Question

The elemental mass percent composition of ascorbic acid (vitamin C) is 40.92% C, 4.58% H, and 54.50% O. Determine the empirical formula of ascorbic acid.

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Textbook Question

A 45.2-mg sample of phosphorus reacts with selenium to form 131.6 mg of the selenide. Determine the empirical formula of phosphorus selenide.