Mole concept empirical formula

Mass and volume relations with emphasis on the mole concept, including empirical formulas and limiting reactants... 

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. 

In the previous section, you learned how to calculate the percentage composition of a compound from its chemical formula. Now you will do the reverse. You will use the percentage composition of a compound, along with the concept of the mole, to calculate the empirical formula of the compound. Since the percentage composition can often be determined by experiment, chemists use this calculation when they want to identify a compound. 

Concept Mapping Design a concept map that illustrates the mole concept. Include moles, Avogadro s number, molar mass, number of particles, percent composition, empirical formula, and molecular formula. 

The empirical formula of a compound can be simply related to the mass percentage of its constituent elements using the mole concept. For example, the empirical formula for ethylene (molecular formula C2H4) is CH2. Its composition by mass is calculated from the masses of carbon and hydrogen in 1 mol of CH2 formula units ... 

As we learned in Section 2.6, the empirical formula for a substance tells us the relative number of atoms of each element in the substance. The empirical formula H2O shows that water contains two H atoms for each O atom. This ratio also applies on the molar level 1 mol of H2O contains 2 mol of H atoms and 1 mol of O atoms. Conversely, the ratio of the numbers of moles of all elements in a compound gives the subscripts in the compound s empirical formula. Thus, the mole concept provides a way of calculating empirical formulas. 

EMPIRICAL FORMULAS FROM ANALYSES We apply the mole concept to determine chemical formulas from the masses of each element in a given quantity of a compound. 

There are several types of calculations that can be undertaken once we recognize the concept that a mole is simply a way to count particles. We can determine the empirical formula of a chemical based on the mass percentage analysis of its elements. We can also determine the molar concentration of solutions prepared by dissolving a known mass of the solute or by diluting a concentrated solution. 

You can also use the mole concept to calculate the empirical formula of a compound using the percentage composition data for that compound — the percentage by weight of each element in the compound. (The empirical formula indicates the different types of elements in a molecule and the lowest whole-number ratio of each kind of atom in the molecule. See Chapter 7 for details.)... 

Before calculating some empirical formulas, it is important to know what is meant by a mole of substance. The mole concept was discussed in detail in Section 2.3 and is summarized below ... 

In Section 3.2, we learned how to use the chemical formula (either molecular or empirical) to determine the percent composition by mass. With the concepts of the mole and molar mass, we can now use the experimentally determined percent composition to determine the empirical formula of a compoimd. Sample Problem 3.8 shows how to do this. 

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