Experimental Determination of Empirical Formulas

The fact that we can determine the empirical formula of a compound if we know the percent composition enables us to identify compounds experimentally. The procedure is as follows. First, chemical analysis tells us the number of grams of each element present in a given amount of a compound. Then, we convert the quantities in grams to number of moles of each element. Finally, using the method given in Example 3.9, we find the empirical formula of the compound. 

The masses of CO2 and of H2O produced can be determined by measuring the increase in mass of the CO2 and H2O absorbers, respectively. Suppose that in one experiment the combustion of 11.5 g of ethanol produced 22.0 g of CO2 and 13.5 g of H2O. We can calculate the mass of carbon and hydrogen in the original 11.5-g sample of ethanol as follows  

5 g of ethanol contains 6.00 g of carbon and 1.51 g of hydrogen. The remainder must be oxygen, whose mass is 

The number of moles of each element present in 11.5 g of ethanol is 

The formula of ethanol is therefore CosoHi 5O025 (we round off the number of moles to two significant figures). Because the number of atoms must be an integer, we divide the subscripts by 0.25, the smallest subscript, and obtain for the empirical formula C2HgO. 

Apparatus for determining the empirical formula of ethanol. The absorbers are substances that can retain water and carbon dioxide, respectively. 

---

As we saw in Section 3.4, knowing the mass of each element contained in a sample of a substance enables us to determine the empirical formula of the substance. One common, practical use of this ability is the experimental determination of empirical formula by combustion analysis. 

Notice that the empirical formula for glucose gives less information than the molecular formula, simply noting the fact that there are twice as many hydrogen atoms as either carbon or oxygen atoms. You may wonder why anyone would be interested in the empirical formula if it contains less information. The answer to that is very simple It is easier to experimentally determine the empirical formula of a compound than the molecular formula of a compound, so that the empirical formula is often found first, as a step towards finding the molecular formula. 

The analyses which follow are arranged in the order in which they would be applied to a newly discovered substance, the estimation of the elements present and molecular weight deter-minations(f.e., determination of empirical and molecular formulae respectively) coming first, then the estimation of particular groups in the molecule, and finally the estimation of special classes of organic compounds. It should be noted, however, that this systematic order differs considerably from the order of experimental difficulty of the individual analyses. Consequently many of the later macro-analyses, such as the estimation of hydroxyl groups, acetyl groups, urea, etc. may well be undertaken by elementary students, while the earlier analyses, such as estimation of elements present in the molecule, should be reserved for more senior students. 

Use the following experimental information to determine the empirical formula of an oxide of silicon. 

From the experimental data, determine the empirical formula of copper(II) chloride, and the error in determining the percent of copper. 

The molecular formula of a compound is always an integer multiple (e.g., 1, 2,3,...) of the empirical formula. If the empirical formula of a compound is known, the molecular formula can be determined by the experimental determination of the molecular weight of the compound... 

The first step is to divide the experimentally determined molecular weight of the compound by the molecular weight of the empirical formula in order to determine the integer multiple that represents the number of empirical formula units in the molecular formula, hi the second step, the molecular formula is obtained by multiplying the subscripts of the empirical formula by the integral multiple of empirical formula units. 

Determine the integer that Experimental molar mass relates the empirical and Mass of empirical formula molecular formulas. ... 

In this experiment you will determine the empirical formula of a compound composed of lead and iodine. A weighed quantity of lead is reacted with nitric acid, HNO3, solution. The resulting lead nitrate solution is then reacted with sodium iodide, Nal, solution to form insoluble lead iodide, which is filtered, dried, and weighed. From your experimental data you can calculate the percentage composition and the ratio of moles of lead to moles of iodine in the compound, and then write the empirical formula. 

Rather than giving students straight percent composition data for determining the empirical formula of a compound (see Question 7), sometimes chemistry teachers will try to emphasize the experimental nature of formula determination by converting the percent composition data into actual experimental... 

Because the percent composition by mass can be obtained experimentally by a number of methods, the procedure outlined in Example 0.16 enables us to determine the empirical formula of an unknown compound. In fact, the word empirical literally means based only on observation and measurement. ... 

The empirical formula of a compound is based on experiments that give the number of moles of each element in a sample of the compound. That is why we use the word "empirical," which means "based on observation and experiment" Chemists have devised a number of different experimental techniques to determine the empirical formulas of compounds. One of these is combustion analysis, which is commonly used for compounds containing principally carbon and hydrogen as their component elements. 

Percent composition data yield only simplest formulas. To determine the molecular formula for a molecular compound, both its empirical formula and its molecular weight must be known. Some methods for experimental determination of molecular weights are introduced in Chapters 12 and 14. 

Once you determine the empirical formula for a compound, you can calculate its empirical formula weight. If you have an experimental determination of its molecular weight, you can calculate n and then the molecular formula. The next example illustrates how you use percentage composition and molecular weight to determine the molecular formula of acetic acid. 

Since the carbon to hydrogen ratio of 1 2.67 is too far from a whole number to be attributed to experimental error, the empirical formula must be the lowest multiple to give a whole number ratio, and 2 x (1 2.67) gives 2 5.34, still not a whole number, but 3 x (1 2.67) gives 3 8.01, which is a whole number ratio within the experimental error. It is difficult to be precise about the experimental error that is acceptable in determinations of this kind, but the percentage of C, H, or N determined by combustion analysis is usually quoted to 0.1%. 

In Section 3.2, we learned how to use the chemical formula (either molecular or empirical) to determine the percent composition by mass. With the concepts of the mole and molar mass, we can now use the experimentally determined percent composition to determine the empirical formula of a compoimd. Sample Problem 3.8 shows how to do this. 

The size of the droplets formed in an aerosol has been examined for a range of conditions important in ICP/MS and can be predicted from an experimentally determined empirical formula (Figure 19.6). Of the two terms in the formula, the first is most important, except at very low relative flow rates. At low relative velocity of liquid and gas, simple droplet formation is observed, but as the relative velocity increases, the stream of liquid begins to flutter and to break apart into long thinner streamlets, which then break into droplets. At even higher relative velocity, the liquid surface is stripped off, and the thin films so-formed are broken down into... 

Experimentally it has been shown that for air-water systems the value of Tj /Zc c, the psychrometric ratio, is approximately equal to 1. Under these conditions the wet-bulb temperatures and adiabatic-saturation temperatures are substantially equal and can be used interchangeably. The difference between adiabatic-saturation temperature and wet-bulb temperature increases with increasing humidity, but this effect is unimportant for most engineering calculations. An empirical formula for wet-bulb temperature determination of moist air at atmospheric pressure is presented by Liley [Jnt. J. of Mechanical Engineering Education, vol. 21, No. 2 (1993)]. 

The empirical formula is C3H2N02, with a formula mass of 84.0 g/mol. This is one-half the experimentally determined molar mass. Thus, the molecular formula is C6H4N204. 

A reasonable empirical formula is C4H4S, which has an empirical mass of 84.1 g/mol. Since this is the same as the experimentally determined molar mass, the molecular formula of thiophene if C4H4S. 

In the next section of Addnl Ref O, Evaluation of Parameters , it is stated that the parameters, y, a, and B/Q, which appear in the LSZK equation of state, must be evaluated by using experimental data and it is explained how this is done on pp 9 10. As an example, compressed TNT of various densities was investigated. Detonation velocities determined by LSZK expression (34) proved to be in good agreement with those detd by the empirical formula ... 

Other early experimenters obtained picric acid by nitrating various organic substances such as silk, natural resins, etc. The correct empirical formula for picric acid was determined by Laurent in 1841 who prepared the acid by reacting phenol with nitric acid and isolated dinit-rophenol which was formed in an intermediate stage of the reaction. 

---