From percent composition

Atoms and their symbols were introduced in Chap. 3 and 1. In this chapter, the representation of compounds by their formulas will be developed. The formula for a compound (Sec. 4.3) contains much information of use to the chemist. We will learn how to calculate the number of atoms of each element in a formula unit of a compound. Since atoms are so tiny, we will learn to use large groups of atoms—moles of atoms—to ease our calculations. We will learn to calculate the percent by mass of each element in the compound. We will learn how to calculate the simplest formula from percent composition data, and to calculate molecular formulas from simplest formulas and molecular weights. The procedure for writing formulas from names or from knowledge of the elements involved will be presented in Chaps. 5. ft. and 13. 

Be able to calculate the empirical formula from percent composition data or quantities from chemical analysis. 

Making particle numbers manageable with Avogadro s number Converting between masses, mole counts, and volumes Dissecting compounds with percent composition Moving from percent composition to empirical and molecular formulas... 

In the previous section the problems presented a chemical formula and asked for the percent composition. There is a method for going from percent composition to chemical formula however, you will obtain only the empirical formula from this. The three steps in determining the empirical formula of a compound from the percent composition are as follows ... 

To determine the molecular formula from percent composition and molecular mass data or from the empirical formula and molecular mass data... 

We can find the empirical formula from percent composition data. The empirical formula represents a ratio therefore, it does not depend on the size of the sample under consideration. Because the empirical formula reflects a mole ratio, and percent composition data are given in terms of mass, we have to convert the masses to moles. We then convert the mole ratio, which is unlikely to be an integral ratio, to the smallest possible whole-number ratio, from which we write the empirical formula. 

The steps we take to obtain an empirical formula from percent composition data are given in the left column (Steps) that follows. In the right column (Example), the empirical formula of a compound containing 39.2% phosphorus and 60.8% sulfur is calculated. 

If we calculated the percent compositions of C2H2 and CeHg (Figure 7.3), we would find that both have the same percentages of carbon and the same percentages of hydrogen (compare Problem 7.100 at the end of the chapter). Both have the same empirical formula—CH. This result means that we cannot tell these two compounds apart from percent composition data alone. However, if we also have a molar mass, we can use that information with the percent composition data to determine not only the empirical formula but also the molecular formula. 

What can we determine from percent composition data ... 

Calculating an Empirical Formula from Percent Composition... 

For more practice calculating an empirical formula from percent composition, go to Supplemental Practice Problems in Appendix A. 

The steps in determining empirical and molecular formulas from percent composition or mass data are outlined below. As in other calculations, the route leads from mass through moles because formulas are based on the relative numbers of moles of elements in each mole of compound. 

In Section 2-10 we distinguished between simplest and molecular formulas of compounds. We showed how simplest formulas can be calculated from percent compositions of compounds. The molecular weight must be known to determine the molecular formula of a compound. For compounds that are gases at convenient temperatures and pressures, the ideal gas law provides a basis for determining molecular weights. 

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. 

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.)... 

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. 

The formula calculated from percent composition by mass is always the empirical formula because the coefficients in the formula are always reduced to the smallest whole numbers. To calculate the actual, molecular formula we must know the approximate molar mass of the compound in addition to its empirical formula. Knowing that the molar mass of a compound must be an integral multiple of the molar mass of its empirical formula, we can use the molar mass to find the molecular formula, as Example 3.11 demonstrates. 

Empirical Formula from Percent Composition Methyl acetate is a solvent commonly used in some paints, inks, and adhesives. Determine the empirical formula for methyl acetate, which has the following chemical analysis ... 

While it is clear that percent composition can be derived from formulas, the reverse is also true. Empirical formulas, or formulas in which the subscripts are in their lowest terms (e.g., acetylene [CjHj] has an empirical formula of CH), can be derived from percent composition data. Knowing what elements are in the compound and having some data about the relative amounts of these elements in the compound, we can determine the subscripts and the empirical formula. 

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