Hydrogen combustion analysis

An 874 mg sample of cortisol was subjected to carbon-hydrogen combustion analysis. 2.23 g of carbon dioxide and 0.652 g of water were produced. The molar mass of cortisol was found to be 362 g/mol using a mass spectrometer. If cortisol contains carbon, hydrogen, and oxygen, determine its molecular formula. 

If 1.00 g of methanol is subjected to carbon-hydrogen combustion analysis, what masses of carbon dioxide and water are produced ... 

Menthol is a compound that contains C, H and O. It is derived from peppermint oil and is used in cough drops and chest rubs. When 0.2393 g of menthol is subjected to carbon-hydrogen combustion analysis, 0.6735 g of C02 and 0.2760 g of H20 are obtained. 

Determine the empirical formula of an organic compound containing carbon, hydrogen, and oxygen by combustion analysis (Example M.4). 

M.12 A compound produced as a by-product in an industrial synthesis of polymers was found to contain carbon, hydrogen, and iodine. A combustion analysis of 1.70 g of the compound produced 1.32 g of C02 and 0.631 g of H20. The mass percentage of iodine in the compound was determined by-converting the iodine in a 0.850-g sample of the compound into 2.31 g of lead(II) iodide. What is the empirical formula of the compound Could the compound also contain oxygen Explain your answer. 

Compounds that do not decompose cleanly into their elements must be analyzed by other means. Combustion analysis is particularly useful for determining the empirical formulas of carbon-containing compounds. In combustion analysis, an accurately known mass of a compound is burned in a stream of oxygen gas. The conditions are carefully controlled so that all of the carbon in the sample is converted to carbon dioxide, and all of the hydrogen is converted to water. Certain other elements present in the sample are also converted to their oxides. 

The assumption is made at present that elemental combustion analysis for carbon, hydrogen, and fluorine provides a good approximation to the extent of incorporation of fluoroalkyl residues, i.e. alcohols and ethers. We have ruled out trifluoromethylcarbonyl groups since no evidence is seen for their presence in either the infrared spectra or the 19F-NMR spectra. Thus, our values for percent modification reflect the best fit of the combustion data to an idealized stoichiometry for the product in Equation 1, where (m+n+o) = 100, and the percent modification (% mod.) is given by the expression [100 x (m+o)/(m+n+o)], equivalent to the number of fluoroalkyl residues per one hundred methylenes. An appropriately normalized formula was used to fit the data for polypropylene (sample 10). 

The determination of the carbon-hydrogen-oxygen content of a compound by combustion analysis involves realizing that all of the carbon has formed carbon dioxide, all of the hydrogen has formed water, and the amount of oxygen present in the original compound must be determined by difference. 

It can be seen that the mass of carbon and hydrogen in the sample does not account for the total mass of the sample that was subjected to combustion analysis. As no other product was formed, the original sample must also have contained oxygen. The mass of oxygen in the original sample can be determined as follows ... 

A historically important form of gravimetric analysis was combustion analysis, used to determine the carbon and hydrogen content of organic compounds burned in excess 02 (Figure 27-4). Instead of weighing combustion products, modem instruments use thermal conductivity, infrared absorption, or coulometiy (with electrochemically generated reagents) to measure the products. 

Figure 27-4 Gravimetric combustion analysis for carbon and hydrogen.

After we receive the results of a combustion analysis from the laboratory, we need to convert the mass percentage composition to an empirical formula. For this step, we need to determine the relative number of moles of each type of atom. The simplest procedure is to imagine that we have a sample of mass 100 g exactly. That way, the mass percentage composition tells us the mass in grams of each element. Then we can use the molar mass of each element to convert these masses into moles and go on to find the relative numbers of moles of each type of atom. Let s do that for vitamin C, which was once identified in this way, and suppose that the laboratory has reported that the sample you supplied is 40.9% carbon, 4.58% hydrogen, and 54.5% oxygen. 

One of the most common methods used to determine percent composition and empirical formulas, particularly for compounds containing carbon and hydrogen, is combustion analysis. In this method, a compound of unknown composition is burned with oxygen to produce the volatile combustion products C02 and H20, which are separated and weighed by an automated instrument called a gas chromatograph. Methane (CH4), for instance, burns according to the balanced equation... 

As an example of how combustion analysis works, imagine that we have a sample of a pure substance—say, naphthalene (often used for household moth balls). We weigh a known amount of the sample, burn it in pure oxygen, and then analyze the products. Let s say that 0.330 g of naphthalene reacts with 02 and that 1.133 g of C02 and 0.185 g of H20 are formed. The first thing to find out is the number of moles of carbon and hydrogen in the C02 and H20 products so that we can calculate the number of moles of each element originally present in the naphthalene sample. 

Worked Example 3.18 shows a combustion analysis when the sample contains oxygen in addition to carbon and hydrogen. Because oxygen yields no combustion products, its presence in a molecule can t be directly detected by this method. Rather, the presence of oxygen must be inferred by subtracting the calculated masses of C and H from the total mass of the sample. 

Caproic acid, the substance responsible for the aroma of dirty gym socks and running shoes, contains carbon, hydrogen, and oxygen. On combustion analysis, a 0.450 g sample of caproic acid gives 0.418 g of H20 and 1.023 g of C02. What is the empirical formula of caproic acid If the molecular mass of caproic acid is 116.2 amu, what is the molecular formula ... 

Coniine, a toxic substance isolated from poison hemlock, contains only carbon, hydrogen, and nitrogen. Combustion analysis of a 5.024 mg sample yields 13.90 mg of C02 and 6.048 mg of H20. What is the empirical formula of coniine ... 

All catalysts were Soxhlet extracted with xylene followed by drying in nitrogen overnight. Carbon, hydrogen, nitrogen and sulfur content were determined by combustion analysis. 

A 0.539 g sample of a compound that contained only carbon and hydrogen was subjected to combustion analysis. The combustion produced 1.64 g of carbon dioxide and 0.807 g of water. Calculate the percentage composition and the empirical formula of the sample. 

Draw a schematic diagram of a carbon-hydrogen combustion analyzer. Write a few sentences to describe each stage of the analysis as dimethyl ether, C2H60, passes through the apparatus. 

A compound that contains carbon, hydrogen, and oxygen is going to be analyzed in a carbon-hydrogen combustion analyzer. Before beginning the analysis, which of the following steps must be carried out ... 

Analytical data obtained from elemental combustion analysis for Fraction 30 of the five SRC samples are presented in Table IV. The relatively constant molecular size of each Fraction 30 sample with 22-28 carbon atoms per average molecule is noteworthy. The only exception which has been noted previously in terms of the mol wt data is the Monterey Fraction 30 sample with 38 carbon atoms per average molecule. The total hydrogen to carbon ratio (H/C)tot is lower for each Fraction 30 sample than (H/C)tot obtained for the non-chromatographed THF-soluble SRC sample. Perhaps one of the more significant trends is the consistently lower (H/C)tot values for the western SRC Fraction 30 (Amax, 0.77 Monterey, 0.79) relative to the other Fraction 30 samples (0.86-0.90). 

Two analytical approaches have been adopted in attempts to obtain stable-isotopic information on insoluble organic matter in carbonaceous chondrites stepped-combustion analysis (e.g., Kerridge, 1983 Swart et al, 1983) and CSIA of pyrolysis products (e.g., Sephton et al, 1998). Stepped-combustion analysis has proved to be more successful in providing information on the major-elemental constituents of chondritic organic matter, i.e., carbon, hydrogen, nitrogen, and oxygen, whereas CSIA has started to yield detailed carbon isotopic and structural information. 

The used catalysts were soxhlet extracted in xylene followed by drying at 200 C. Carbon, hydrogen and nitrogen contents were determined by combustion analysi.s. 

Elemental Analysis The carbon, hydrogen, and nitrogen contents of reference standards may be determined by combustion analysis. Combustion analysis thus provides an orthogonal determination of the molecular formula and the purity of the reference standard. Poor agreement between the theoretical and experimental elemental compositions is generally an indication of an impure substance or a discrepancy between the theoretical and actual molecular formulas. Elemental analysis may also identify the presence of solvates and inorganic contaminants. Elemental analysis is typically performed in specialized laboratories under blinded conditions. 

Table 6.13 Site-specific stable hydrogen isotope analysis of linalylacetate by SNIF-NMR. Adapted from [230] with kind permission. Copyright [1992] American Chemical Society. The overall D/H- and ( C/ C)-isotope ratios are determined by combustion and IRMS. S C-values are expressed in [%c[ V5. V-PDB, the overall D/H- and the site-specific D/H-ratios in parts per million [ppm], position indication see Fig. 6.22 (D/H) is the methyl group of the acetyl residue in this case...

The first piece of information we try to get is the molecular formula from the mass spectral formula weight and the percent composition from a combustion analysis if available. If we don t have a molecular formula, we can still get a minimum count on the number of carbons and hydrogens from the NMR spectra, which will be discussed later. 

Combustion analysis 66.63% carbon and 11.18% hydrogen does not sum to 100% therefore the remaining 22.19% must be oxygen. Also, no other elements are present. 

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