Determination of Empirical and Molecular Formulas

By this point in your study of chemistry, you have seen hundreds of chemical formulas. Have you wondered where they come from or how we know the relative numbers of atoms of each element in a compound This section describes some of the ways chemists determine chemical formulas from experimental data. 

Before beginning, we need to understand the distinction between two types of chemical formulas, empirical formulas and molecular formulas. When the subscripts in a chemical formula represent the simplest ratio of the kinds of atoms in the compound, the formula is called an empirical formula. Most ionic compounds are described with empirical formulas. For example, chromium(III) oxide s formula, Cr203, is an empirical formula. The compound contains two chromium atoms for every three oxide atoms, and there is no lower ratio representing these relative amounts. 

Molecular compounds are described with molecular formulas. A molecular formula describes the actual numbers of atoms of each element in a molecule. Some molecular formulas are also empirical formulas. For example, water molecules are composed of two hydrogen atoms and one oxygen atom, so water s molecular formula is H2O. Because this formula represents the simplest ratio of hydrogen atoms to oxygen atoms in water, it is also an empirical formula. 

If we know the relative mass or the mass percentage of each element in a compound, we can determine the compound s empirical formula. The general procedure is summarized in Study Sheet 9.2, hut before we look at it, let s reason it out using a substance that is sometimes called photophor, an ingredient in signal fires, torpedoes, fireworks, and rodent poison. 

The subscripts in an empirical formula are positive integers representing the simplest ratio of the atoms of each element in the formula. For now, we can describe the empirical formula for photophor in the following way  

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The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

Introduction to organic chemistry hydrocarbons and functional groups (structure, nomenclature, chemical properties). Physical and chemical properties of simple organic compounds should also be included as exemplary material for the study of other areas such as bonding, equilibria involving weak acids, kinetics, colligative properties, and stoichiometric determinations of empirical and molecular formulas. 

M.9 The stimulant in coffee and tea is caffeine, a substance of molar mass 194 g-mol When 0.376 g of caffeine was burned, 0.682 g of carbon dioxide, 0.174 g of water, and 0.1 10 g of nitrogen were formed. Determine the empirical and molecular formulas of caffeine, and write the equation for its combustion. 

This is a critical chapter in your study of chemistry. Our goal is to help you master the mole concept. You will learn about balancing equations and the mole/mass relationships (stoichiometry) inherent in these balanced equations. You will learn, given amounts of reactants, how to determine which one limits the amount of product formed. You will also learn how to determine the empirical and molecular formulas of compounds. All of these will depend on the mole concept. Make sure that you can use your calculator correctly. If you are unsure about setting up problems, refer back to Chapter 1 of this book and go through Section 1-4, on using the Unit Conversion Method. Review how to find atomic masses on the periodic table. Practice, Practice, Practice. 

M.10 The bitter-tasting compound quinine is a component of tonic water and is used as a protection against malaria. When a sample of mass 0.487 g was burned, 1.321 g of carbon dioxide, 0.325 g of water, and 0.0421 g of nitrogen were produced. The molar mass. of quinine is 324 g-mol-1. Determine the empirical and molecular formulas of quinine. 

Cumene is a compound containing only carbon and hydrogen that is used in the production of acetone and phenol in the chemical industry. Combustion of 47.6 mg of cumene produces some C02 and 42.8 mg of water. The molar mass is between 115 and 125 g. Determine the empirical and molecular formulas. 

A compound contains only C, H, and N. It is 58.51% C and 7.37% H by mass. Helium effuses through a porous frit 3.20 times faster than the compound does. Determine the empirical and molecular formulas of this compound. 

Anthraquinone contains only carbon, hydrogen, and oxygen. When 4.80 mg of anthraquinone is burned, 14.22 mg of C02 and 1.66 mg of H20 are produced. The freezing point of camphor is lowered by 22.3°C when 1.32 g of anthraquinone is dissolved in 11.4 g of camphor. Determine the empirical and molecular formulas of anthraquinone. 

Succinic acid is a substance produced by lichens. Chemical analysis indicates it is composed of 40.68% carbon, 5.08% hydrogen, and 54.24% oxygen and has a molar mass of 118.1 g/mol. Determine the empirical and molecular formulas for succinic acid. 

Naphthalene, commonly known as moth balls, is composed of 93.7% carbon and 6.3% hydrogen. The molar mass of napthalene is 128 g/mol. Determine the empirical and molecular formulas for naphthalene. 

When 1.125 g of a liquid hydrocarbon, CJTj, was burned in an apparatus like that in Figure 4.12, 3.447 g of CO2 and 1.647 g of H2O were produced. The molar mass of the compound was found to be 86.2 g/mol in a separate experiment. Determine the empirical and molecular formulas for the unknown hydrocarbon, GJT. ... 

I Explain what is meant by the percent composition of a compound. I Determine the empirical and molecular formulas for a compound from mass percent and actual mass data. 

A compound having an approximate molar mass of 165-170 g has the following percentage composition by mass carbon, 42.87% hydrogen, 3.598% oxygen, 28.55% nitrogen, 25.00%. Determine the empirical and molecular formulas of the compound. 

Determine the empirical and molecular formulas of each of the following substances ... 

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