Mass relationships in chemical reactions

Sulfur burning in oxygen to form sulfur dioxide. The models show elemental sulfur (Sg), and oxygen and sulfur dioxide molecules. About 50 million tons of SO2 are released to the atmosphere every year. 

In this chapter we will consider the masses of atoms and moleeules and what happens to them when chemical changes occur. Our guide for this diseussion will be the law of conservation of mass. 

Section 3.4 describes a method for determining atomic mass. 

One atomic mass unit is also called one dalton. 

In this chapter, we will use what we have learned about chemical structure and formulas in studying the mass relationships of atoms and molecules. These relationships in turn will help us to explain the composition of compounds and the ways in which composition changes. 

A balanced chemical equation makes a quantitative statement about the relative masses of the reacting substances. The chemical equation for the combustion of butane, 

Divide by molar mass of first reactant or product 

Multiply by chemical conversion factor from balanced equation 

Multiply by molar mass of second reactant or product 

FIGURE 2.4 The steps in a stoichiometric calculation. In a typical calculation, the mass of one reactant or product is known and the masses of one or more other reactants or products are to be calculated using the balanced chemical equation and a table of relative atomic masses. 

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I he previous chapters showed how the laws of conservation of mass and con--1- servation of atomic identity, together with the concept of the mole, determine quantitative mass relationships in chemical reactions. That discussion assumed prior knowledge of the chemical formulas of the reactants and products in each equation. The far more open-ended questions of which compounds are found in nature (or which can be made in the laboratory) and what types of reactions they undergo now arise. Why are some elements and compounds violently reactive and others inert Why are there compounds with chemical formulas H2O and NaCl, but never H3O or NaCli Why are helium and the other noble gases monatomic, but molecules of hydrogen and chlorine diatomic All of these questions can be answered by examining the formation of chemical bonds between atoms. 

Mass relationships in chemical reactions confirm the law of conservation of mass. 

The formula mass in grams of any element or compound must contain the same number of fundamental units as the formula mass of any other element or compund. Chemists call that formula mass, when expressed in grams, the mole. The unit for the mole is mol, introduced in Chap. 1, Table 1-1. The mole has two meanings (1) one formula mass of an element or compound, and (2) a unique number of fundamental units of that element or compound. That number has been determined by experiment to be 6.02205 x lO mol , and has been given the name Avogadro s number, in honor of Amadeo Avogadro, whose ideas were used to solve many of the problems involved in mass relationships in chemical reactions. 

We are now in a position to consider the mass relationships in chemical reactions, or what is known as stoichiometry. Let us examine the quantitative aspects of the reaction in which ammonia is formed from nitrogen and hydrogen ... 

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