Ch.3 - Chemical Reactions and Reaction Stoichiometry

Problem 57a

Chapter 3, Problem 57a

Valproic acid, used to treat seizures and bipolar disorder, is composed of C, H, and O. A 0.165-g sample is combusted to produce 0.166 g of water and 0.403 g of carbon dioxide. What is the empirical formula for valproic acid?

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Determine the mass of hydrogen in the water produced by using the fact that water is composed of hydrogen and oxygen in a known mass ratio. For every 18 g of water, there are 2 g of hydrogen. Use this ratio to calculate the mass of hydrogen in the 0.166 g of water produced.

Calculate the mass of carbon in the carbon dioxide produced. Carbon dioxide is composed of carbon and oxygen, and for every 44 g of CO2, there are 12 g of carbon. Use this ratio to find the mass of carbon in the 0.403 g of carbon dioxide produced.

Calculate the mass of oxygen in the valproic acid sample by subtracting the mass of hydrogen and carbon (obtained from the previous steps) from the total mass of the valproic acid sample.

Convert the masses of carbon, hydrogen, and oxygen to moles by using their respective molar masses (C: 12.01 g/mol, H: 1.008 g/mol, O: 16.00 g/mol).

Divide the number of moles of each element by the smallest number of moles to find the simplest whole number ratio of C, H, and O in the empirical formula of valproic acid.

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Key Concepts

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

Combustion Analysis

Combustion analysis is a technique used to determine the elemental composition of organic compounds. In this process, a sample is burned in excess oxygen, producing carbon dioxide and water. The masses of these products are measured to calculate the amounts of carbon, hydrogen, and sometimes oxygen in the original sample.

Mole Concept

The mole concept is a fundamental principle in chemistry that relates the mass of a substance to the number of particles it contains. One mole of any substance contains Avogadro's number (approximately 6.022 x 10²³) of entities, whether they are atoms, molecules, or ions. This concept is essential for converting grams to moles, which is necessary for determining the empirical formula.

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 amounts of each element calculated from combustion data. The empirical formula does not necessarily reflect the actual number of atoms in a molecule but provides a basic understanding of the compound's composition.

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Textbook Question

(b) Menthol, the substance we can smell in mentholated cough drops, is composed of C, H, and O. A 0.1005-g sample of menthol is combusted, producing 0.2829 g of CO₂ and 0.1159 g of H₂O. What is the empirical formula for menthol? If menthol has a molar mass of 156 g/mol, what is its molecular formula?

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Textbook Question

(a) The characteristic odor of pineapple is due to ethyl butyrate, a compound containing carbon, hydrogen, and oxygen. Combustion of 2.78 mg of ethyl butyrate produces 6.32 mg of CO₂ and 2.58 mg of H₂O. What is the empirical formula of the compound?

(b) Nicotine, a component of tobacco, is composed of C, H, and N. A 5.250-mg sample of nicotine was combusted, producing 14.242 mg of CO₂ and 4.083 mg of H₂O. What is the empirical formula for nicotine? If nicotine has a molar mass of 160 ± 5 g/mol, what is its molecular formula?

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Textbook Question

Propenoic acid, C₃H₄O₂, is a reactive organic liquid that is used in the manufacturing of plastics, coatings, and adhesives. An unlabeled container is thought to contain this liquid. A 0.275-g sample of the liquid is combusted to produce 0.102 g of water and 0.374 g carbon dioxide. Is the unknown liquid propenoic acid? Support your reasoning with calculations.

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Textbook Question

Washing soda, a compound used to prepare hard water for washing laundry, is a hydrate, which means that a certain number of water molecules are included in the solid structure. Its formula can be written as Na2CO3 # xH2O, where x is the number of moles of H2O per mole of Na2CO3. When a 2.558-g sample of washing soda is heated at 125 C, all the water of hydration is lost, leaving 0.948 g of Na2CO3. What is the value of x?

Epsom salts, a strong laxative used in veterinary medicine, is a hydrate, which means that a certain number of water molecules are included in the solid structure. The formula for Epsom salts can be written as MgSO4 # xH2O, where x indicates the number of moles of H2O per mole of MgSO4. When 5.061 g of this hydrate is heated to 250 C, all the water of hydration is lost, leaving 2.472 g of MgSO4. What is the value of x?

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