Empirical formulas for ionic compound

For transition metals, the lanthanides, and the actinides, no such simple rule exists. If we accept the ions charges as chemical facts, we can still write the empirical formulas for ionic compounds so that the net (overall) charge is zero. If we had Fe2+ and O2-, the compound would require a minimum of one of each of the elements for a neutral formula, FeO, whereas Fe3+ and O2- would have the formula Fe2C>3. 

Using Ionic Charge to Write Empirical Formulas for Ionic Compounds... 

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. 

An ionic compound sometimes called pearl ash is used to make special glass for color TV tubes. A sample of this compound is analyzed and found to contain 56.50% potassium, 8.75% carbon, and 34.75% oxygen. What is the empirical formula for this compound What is its chemical name ... 

An ionic compound used in the brewing industry to clean casks and vats and in the wine industry to kill undesirable yeasts and bacteria is composed of 35.172% potassium, 28.846% sulfur, and 35.982% oxygen. What is the empirical formula for this compound ... 

An ionic compound that contains 10.279% calcium, 65.099% iodine, and 24.622% oxygen is used in deodorants and in mouthwashes. What is the empirical formula for this compound What do you think its chemical name is (Consider the possibility that this compound contains more than one polyatomic ion.)... 

The ionic compound sometimes called uranium yellow is used to produce colored glazes for ceramics. It is 7.252% sodium, 75.084% uranium, and 17.664% oxygen. Wliat is the empirical formula for this compound ... 

There are two broad classes of formulas for compounds empirical formulas and molecular formulas. The empirical formula shows the simplest ratio of elements in a compound and uses the smallest possible set of subscript numbers. Empirical formulas are also called simple formulas. The formulas of a// ionic compounds are empirical formulas. Since ionic compounds do not exist as molecules, their formulas are not molecular formulas. The formulas calculated from percent composition data are empirical formulas. (We ll get to these calculations later.)... 

The formulas of ionic compounds are always the same as their empirical formulas because ionic compounds do not consist of discrete molecular units. For example, a solid sample of sodium chloride (NaCl) consists of equal numbers of Na and Cl ions arranged in a three-dimensional network (Figure 2.11). In such a compound there is a 1 1 ratio of cations to anions so that the compound is electrically neutral. As you can see in Figure 2.11, no Na" ion in NaCl is associated with just one particular d ion. In fact, each Na ion is equally held by six surrounding Cl ions and vice versa. Thus NaCl is the empirical formula for sodium chloride. In other ionic compounds the actual structure may be different, but the arrangement of cations and anions is such that the compounds are all electrically neutral. Note that the charges on the cation and anion are not shown in the formula for an ionic compound. 

Consider Figure 4.19 in the text. Why is it that the formulas for ionic compound are always empirical formulas ... 

Ionic compounds are compounds that are formed from cations and anions. Such compounds are held together by the electrostatic attraction between opposite charges. An example of an ionic compound is sodium chloride (NaCl), common table salt. The formulas of ionic compounds are usually the same as their empirical formulas because ionic compounds do not consist of discrete molecular units. For example, a solid sampk dium cl iid j((NagJ ccn s numbers of Na and Q ions... 

The formula of an ionic compound is always an empirical formula, for example the empirical formula of aluminium oxide is AI2O3. 

Many compounds in nature, particularly compounds made of Ccirbon, hydrogen, and oxygen, are composed of atoms that occur in numbers that are multiples of their empirical formula. In other words, their empirical formulas don t reflect the actual numbers of atoms within them instead, they reflect only the ratios of those atoms. What a nuisance Fortunately, this is an old nuisance, so chemists have devised a means to deal with it. To account for these annoying types of compounds, chemists are Ccireful to differentiate between an empirical formula and a moleculcir formula. A molecular formula uses subscripts that report the actual number of each type of atom in a molecule of the compound (a formula unit accomplishes the same thing for ionic compounds). 

Molecular formulas give all the information that empirical formulas do, plus the ratio of the number of moles of each element to the number of moles of the compound. (Molecular formulas are used only for molecular substances, not ionic substances.) A molecular formula can be determined from the empirical formula of the compound and its formula mass First, divide the formula mass by the mass in amu of one empirical formula unit, which will result in a small integer. Then, multiply each subscript of the empirical formula by that integer. (Section 7.5)... 

Ionic compounds, however, are composed of ions, not covalently bonded atoms. For ionic compounds formula mass should be used instead of molecular mass and empirical formula, simplest formula or formula unit should be used instead molecular formula. 

An empirical formula shows the simplest ratio of atoms in a molecule (or an ionic compound). For a molecular substance, the empirical formula only tells you the ratio of atoms, so it may not be very helpful at identifying the nature of a substance. For example, take any of the group of hydrocarbons known as the alkenes. All of the different compounds have a l-to-2 ratio of carbon to hydrogen. Ethene is C2H4, propene is C H, and butene is C,Hj. If you reduce all of the subscripts, they each come out to CH2. Knowing the empirical formula helps to tell you that this substance is an alkene, but it doesn t tell you which one. How can you determine what a compound is from the empirical formula The molecular weight will solve your problem. If you know the empirical formula and the molecular weight, you can determine the molecular formula for a compound. The first type of calculation we will look at is the determination of the empirical formula. The second type is the determination of a molecular formula. 

What is the chemical formula for this compound You have learned that the formula for an ionic compound represents the simplest possible ratio of the ions present and is called a formula unit. Chemical formulas for most ionic compounds are the same as their empirical formulas. Because the unknown compound is ionic, the chemical formula for a formula unit of the compound is the same as its empirical formula, NaC104. The compound is called sodium perchlorate. 

If you know the formula of a compound, you can calculate its percent composition. Just the reverse can be done too. If you know the percent composition of a compound, you can calculate a formula for the compound. A formula calculated from percent composition data is called an empirical formula (one calculated from experimental data). The formulas of ionic compounds are always empirical formulas. The formulas of molecular compounds may be the same as their empirical formulas or they may be some whole-number multiple of it. You will learn how to do composition-from-fbrmula and fbrmula-from-composition calculations in this chapter. 

It is important to realize that only empirical formulas are calculated from percent composition data or mass data. That s fine for ionic compounds because their formulas are always empirical formulas. But the complete formulas of molecular compounds, the molecular formulas, can only be obtained from the empirical formulas if the molecular mass or molar mass of the compound is also known. 

In order for ionic compounds to be electrically neutral, the sum of the charges on the cation and anion in each formula unit must be zero. If the charges on the cation and anion are numerically different, we apply the following rule to make the formula electrically neutral The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation. If the charges are numerically equal, then no subscripts are necessary. This rule follows from the fact that because the formulas of ionic compounds are empirical formulas, the subscripts must always be reduced to the smallest ratios. Let us consider some examples. 

The ions in ionic compounds are arranged in three-dimensional structures, as Figure 2.21(b) shows for NaCl. Because there is no discrete molecule of NaCl, we are able to write only an empirical formula for this substance. This is true for most other ionic compounds. 

We can write the empirical formula for an ionic compound if we know the charges of the ions. This is true because chemical compounds are always electrically neutral. Consequently, the ions in an ionic compound always occur in such a ratio that the total positive charge equals the total negative charge. Thus, there is one Na to one Cl (giving NaCl), one Ba to two Cl (giving BaCl2), and so forth. 

O (by mass), (a) What is the empirical formula for Compound 1 (b) What is the empirical formula for Compound 2 (c) Upon determining the melting points of these two compounds, you find that the yellow compound melts at 25 °C, while the black powder does not melt up to the maximum temperature of your apparatus, 1200 °C. What is the identity of the yellow compound What is the identity of the black compound Be sure to use the appropriate naming convention depending on whether the compound is better described as a molecular or ionic compound. 

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