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Compounds were defined in Chapter 1 as pure substances that can be chemically separated into elements, although always with specific ratios. This observation is called the law of definite proportions. It was also observed that when elements combine to form compounds, the ratio may be different, depending on the type of compound. This is the law of multiple proportions. Both observations were explained by Dalton's atomic theory. This theory states that atoms, the smallest form of an element, combine in simple whole-number ratios to form molecules, the smallest form of a compound.

The expression that shows the number and kind of each atom in a molecule is the chemical formula. Chemical formulas use the chemical symbol of the atom to express the type of atom and a subscript following the symbol to express the number of atoms in the molecule. If there is only one of a type of atom, no subscript is used. In addition to the number and type of atom, chemical formulas can (but don't have to) suggest the arrangement of the atoms.

Each compound has not only a formula, but also a name. Like the formulas, the name of the compound must provide sufficient information to identify the number and type of each atom in a molecule. Therefore, systems for naming have been developed. The type of system depends on the type of compound. For binary molecular compounds, which are normally comprised of two types of nonmetal atoms, there is a prefix on the name of the element to express the number of that type of atom. If there is only one of that type of atom, no prefix is used. The second element has its ending changed to ide. For binary ionic compounds, usually comprised of a metal and a nonmetal, the cation, then the anion are named. Cations are simply the name of the element for group 1A and 2A metals. For other metals, the name of the metal is followed by its charge in parentheses. The charge is written as a Roman numeral. Monatomic (one-atom) anions change the end of the element name to ide. For polyatomic (multiatom) anions, the system is complex, so that it is generally easier to memorize the names of the common ones (Table 4.1) than to decipher the system. The number of cations and anions is not part of the name of binary ionic compounds. Since the net charge of the positively charged cations must exactly cancel that of the negatively charged anions, the ratio of cations to anions can be deduced from the charges.

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For acids, which produce hydrogen ions in water and normally have hydrogen as the first element in their formula, the naming system depends on the type of acid. Acids made from oxoanions (anions containing oxygen) change the suffix of the oxoanion and add the word acid. Binary acids combine hydrogen with a halogen (group 7A). These acids are named as hydro{halogen root}ic acid. For example, HCl is hydrochloric acid. The last naming system is for organic (carbon-based) compounds. That naming system will not be addressed in this chapter.

The chemical formula tells the type and the ratio of atoms in a compound. Because the ratio is the same regardless of whether there is one molecule or millions, we use a "scale-up" factor to work with enough molecules to be easily manipulated. This factor, the SI unit for amount of substance, is a mole. The mole is just a specific number of things, in the case of chemistry either molecules or atoms. Because atoms and molecules are so small, this amount must be very large. The number of things in a mole is called Avogadro's number and has a value of 6.022 x 10²³. This number is used because whereas the mass of an atom is its atomic weight in atomic mass units, the mass of a mole of atoms is its atomic weight in grams! Since masses are additive, the mass of a molecule is the sum of the mass of each of its atoms in atomic mass units and the mass of a mole of the molecule is the sum of the mass of each of its atoms in grams. The mass of a mole of any substance is called molar mass. Also, since the ratios are the same in a mole as in a molecule, the molecular formula can be used to obtain mole ratios of elements in a compound.

                              >> Explore: Avogadro's Number Tutorial

Consequently, chemical formulas can be used to determine the percent composition, by mass, of a compound.

% composition = (grams of element/grams of compound)100

Since percent is also a ratio, it will be the same for any amount of substance. A convenient amount for these calculations is 1 mole. In 1 mole the grams of compound are equal to the molar mass of the compound. In addition, the grams of element are the mass of the element in one mole of compound. This process uses the mole ratio of the chemical formula to obtain the mass ratio of the percent composition. The process can also be done in reverse. That is, the percent composition can be used to determine the chemical formula.

                              >> Explore: Percent Composition Tutorial

When determining mole ratio (or chemical formula), it is convenient to assume there is 100 g of compound (since another way of looking at percent is grams of element in 100 g of compound). If there are 100 g of compound, the grams of each element are the same as the percentage of each element. The molar mass is used to convert the gram value into moles. The mole values can be used to determine mole ratios that then become the chemical formula. This process is often used for an unknown compound, since experiments can be used to determine percent composition. Therefore chemical formulas determined with this process are called empirical formulas. Empirical means determined by experiment (as opposed to theory). However, this method only determines the simplest ratio of elements in a compound. It turns out that many molecules have atoms that are not in the simplest ratio. To determine the true molecular formula, more information is needed. That extra information is the molar mass. Since the molar mass contains the mass of all elements in a compound, the ratio of the molar mass of the true formula to the molar mass of the empirical formula provides the multiplication factor to determine the true molecular formula.

When substances mix with other substances, the atoms can rearrange to form new substances. This is called a chemical reaction. Chemists express what occurs in a chemical reaction using a chemical equation. Chemical equations list the formulas of starting materials, reactants, on the left (in no particular order) and use an arrow pointing to the list of formulas of the substances created, products, on the right. The law of conservation of mass says that atoms can neither be created nor be destroyed in a chemical reaction. Therefore our chemical equations must also account for each atom and there must be the same number of each type of atom on each side of the arrow. When each atom is accounted for, the equation is balanced. Since changing the subscripts of the chemical formulas would change the type of compound, equations are balanced by describing how many of each type of compound are used or produced. To accomplish this, a number in front of the chemical formula, called the stoichiometric coefficient, is used.

                              >> Explore: Balancing Equations Tutorial

A balanced chemical equation describes both the ratio of molecules needed for a chemical reaction and the ratio of moles needed for a chemical reaction. It can describe the ratio of moles required of the reactants and the ratio of moles of reactants to moles of products. If the number of moles is known, molar mass can be used to determine the number of grams needed. Consequently, chemical equations can be used to determine any number of relationships between reactants and products. The process of doing so is called stoichiometry. It is one of the most important skills learned in introductory chemistry courses.

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Although the chemical reaction can describe the ratio of reactants needed, that is not the same as the amount of reactants available. Since all reactants must be present for the reaction to occur, if one of the reactants gets used up, the reaction will stop regardless of how much of the other reactants may be available. The reactant that gets used up first is called the limiting reactant. Since the reaction stops when the limiting reactant is gone, the limiting reactant determines how much product is made.

                              >> Explore: Limiting Reactant Tutorial

The amount of product made in a chemical reaction is called the yield. Yields are normally expressed as mass (grams). The yield predicted from stoichiometric calculation is called the theoretical yield, since it is based on our stoichiometric theory. Unfortunately, real life is never quite as good as theory. In real chemistry the reactants are sometime used up by other, competing reactions. Sometimes the reactions are just so slow that waiting a million years or so for the reaction to finish is impractical. Also, if atoms can rearrange to form products, what is to stop the atoms of the products from rearranging back into reactants? It turns out that some reactions never reach completion because the reactants get reformed. Consequently, the actual yield, the amount of product really produced by the reaction, is never more than the theoretical yield. How much less can be expressed as percent yield.

% yield = (actual yield/theoretical yield)100