Empirical formulas of organic compounds

In Section F, we saw that one technique used in modern chemical laboratories to determine the empirical formulas of organic compounds is combustion analysis. We are now in a position to understand the basis of the technique, because it makes use of the concept of limiting reactant. 

In Section F, we remarked that one technique used in modern chemical laboratories or the agencies that carry out contract work on behalf of other chemists is combustion analysis. This technique—which has been used since the earliest days of chemistry—is used to establish the empirical formulas of organic compounds and, in combination with mass spectrometry, their molecular formulas. It is used both for newly synthesized compounds and to identify active compounds in natural products. We are now in a position to understand the basis of the technique, for it makes use of the concept of limiting reactant. 

In a similar fashion, quantitative elemental analysis for carbon, hydrogen, nitrogen, and oxygen enabled the chemist to determine the empirical formulas of organic compounds. An empirical formula is the simplest whole number ratio of the atoms of each element present in a molecule. For any given compound, the empirical formula may or... 

A sample of an organic compound containing C, H, and O, which weighs 12.13 mg, gives 30.6 mg of C02 and 5.36 mg of H20, n combustion. The amount of oxygen in the original sample is obtained by difference. Determine the empirical formula of this compound. 

In another experiment an unknown organic compound was found to contain 0.12 g of carbon and 0.02 g of hydrogen. Calculate the empirical formula of the compound. (Ar H = 1 C = 12)... 

Elementary analysis showed that an organic compound contained C, H, N, and O as its only elementary constituents. A 1.279-g sample was burned completely, as a result of which 1.60g of CO2 and 0.77 g of H2O were obtained. A separately weighed 1.625-g sample contained 0.216 g nitrogen. What is the empirical formula of the compound ... 

In addition to using the absolute intensities of the atomic emission lines, the peak intensity ratios of these lines have been used to analyze samples. Tran et al. [77] analyzed the atomic intensity ratios of several organic compounds with the hope to determine the empirical formula of a compound based on the ratios from several elements. Calibration curves were built based on C H, C 0, and C N atomic emission ratios from various compounds that covered a wide range of stoichiometries. Then, four compounds with known stoichiometries were tested against the calibration curves. The ratios determined from the calibration curves were compared with the actual stoichiometries and showed accuracy of 3% on average. In the study of nitroaromatic and polycyclic aromatic hydrocarbon samples, the ratios between C2 and CN and between O and N of different samples were shown to correlate with the molecular formula [75], Anzano et al. [71] also attribute success of their correlation of plastics to differences in the C/H atomic emission intensity ratio of each sample. 

Benzene is the oldest known organic compound, firstly discovered by Michael Faraday in 1825. When he examined the white precipitates in gas tubes used for illumination, he discovered a new compound. He determined the empirical formula of this compound as CH and named it bicarburet of hydrogen . 

An organic compound was analyzed by combustion, and it was found that a sample weighing 0.200 g produced 0.389 g of carbon dioxide and 0.277 g of water. Another sample, weighing 0.150 g, was found on combustion to produce 37.3 ml of nitrogen at standard conditions. What is the empirical formula of the compound ... 

Nitrogen by the Kjeldahl Method.— The absolute method, while the more accurate and often used when the results of analysis are for the purpose of determining the empirical formula of a compound, is sometimes replaced by the Kjeldahl or wet combustion process. In outline the method is as follows The organic compound is decomposed and oxidized by heating for some time in about 30 cc. of pure con-... 

Often, one of the the first steps in the identification of an unknown organic compound is to submit the compound for quantitative elemental analysis. This type of analysis will determine the percentage by mass of the elements present in the compound. From the results of the analysis, the empirical formula of the compound can be calculated. To find out the relative amounts of carbon and hydrogen in a hydrocarbon, a weighed sample of the hydrocarbon is passed through a tube packed with copper(ll) oxide at a temperature of about 700°C. The copper(ll) oxide oxidizes the carbon in the hydrocarbon to carbon dioxide and the hydrogen to steam ... 

A gaseous organic compound containing only carhon, hydrogen, and nitrogen is burned in oxygen gas, and the volume of each reactant and product is measured under the same conditions of temperature and pressure. Reaction of four volumes of the compound produces four volumes of CO2, two volumes of N2, and ten volumes of water vapor, (a) How many volumes of O2 were required (b) What is the empirical formula of the compound ... 

Therefore in order to calculate the molecular formula of a compound it is necessary to know its molar mass. Molar masses can be determined by a variety of physical measurements, including back titrations (for weak acids and bases) and weighing gases. Mass spectrometry is frequently used to determine the molar masses of molecular substances (Chapter 2). Automated instruments for determining the empirical and molecular formulas of organic compounds are available. 

An organic compound sometimes used in the manufacture of perfumes is 29.78% carbon, 4.17% hydrogen, and 66.05% bromine, by mass. What is the empirical formula of this compound ... 

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. 

Table 1.14 Empirical Formula Index of Organic Compounds 1.58...

---