Empirical formulas from percent composition

The first step is to determine the empirical formula from the percent composition data. 

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

To determine an empirical formula from a percent composition, do the following ... 

If 1.00 g of the unknown contains 0.817 g carbon, the mass percent of carbon is 81.7 percent, leaving the remaining 18.3 percent as hydrogen. Therefore, we need to use the procedures for determining an empirical formula from a percentage composition. The problem will progress as follows (remember with percents, assume a 100 g sample) ... 

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 ... 

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. 

We can obtain an empirical formula from mass data instead of a percent composition. 

Determine an empirical formula from each of the following sets of 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. 

To find the empirical formula from the percent mass composition, you assume that you have a 100 gram sample. Now the percent translates directly to grams. 

H.21 (a) We can find the empirical formulas from the percent compositions. 

J.9 Since X turns litmus red and conducts electricity poorly, it is a weak acid. We can find the empirical formula from the percent composition. 

The empirical formula of a compound can be determined if the percent composition of the compound is known (Section 3.7). But where do the percent composition data come from Various methods are used, and many depend on reactions that decompose the unknown but pure compound into known products. Assuming the reaction products can be isolated in pure form, the masses and the number of moles of each can be determined. Then, the moles of each product can be related to the number of moles of each element in the original compound. One method that works well for compounds that burn in oxygen is analysis by combustion. Each element (except oxygen) in the compound combines with oxygen to produce the appropriate oxide. 

Peroxyacylnitrate (PAN) is one of the components of smog. It is a compound of C, H, N, and O. Determine the percent composition of oxygen and the empirical formula from the following percent composition by mass 19.8 percent C, 2.50 percent H, 11.6 percent... 

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 ... 

EMPIRICAL FORMULAS FROM ANALYSIS (SECTION 3.5) The empirical formula of any substance can be determined from its percent composition by calculating the relative number of moles of each atom in 100 g of the substance. If the substance is molecular in nature, its molecular formula can be determined from the empirical formula if the molecular weight is also known. Combustion analysis is a special technique for determining the empirical formulas of compounds containing only carbon, hydrogen, and/or oxygen. 

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