Methane relative molecular mass

The density of a vapour or gas at constant pressure is proportional to its relative molecular mass and inversely proportional to temperature. Since most gases and vapours have relative molecular masses greater than air (exceptions include hydrogen, methane and ammonia), the vapours slump and spread or accumulate at low levels. The greater the vapour density, the greater the tendency for this to occur. Gases or vapours which are less dense than air can, however, spread at low level when cold (e.g. release of ammonia refrigerant). Table 6.1 includes vapour density values. 

Figure 9-8 Diffusion coefficients of organic compounds in LDPE depending on their relative molecular mass, Mr. C-6 to C-24 are hexane to tetracosan 1 undecalactone, 2 phenylethyl alcohol. 3 isoamyl acetate. 4 diphenyl methane, 5 diphenyl ether, A 7-OH to 18-OH are alcohols (heptanol to 1-octadecanol).

The empirical formula of a compound is the simplest whole number ratio of the atoms it contains (Chapter 1). For some alkanes, methane and propane for instance, the empirical formula is the same as the actual molecular formula. However, for others this is not true. The empirical formula of ethane, whose molecular formula is C2Hg, is CHj. In practical terms the empirical formula is the formula that can be derived from the percentage composition data obtained from combustion analysis. In order to use this to establish the actual formula, data on the relative molecular mass (M ) of the compound is required. 

Fig. 11.3. Electron ionization and methane Cl mass spectra of toluene. The key features of the respective mass spectra are labeled. Spectral interpretation is based on recognition and understanding of these key features and how they correlate with structural elements of the analyte molecule of interest. The signal representing the most abundant ion in a mass spectrum is referred to as the base peak, and may or may not be the molecular ion peak (which carries the molecular mass information). Cl spectra provide confirmation of molecular mass in situations where the El signal for the molecular ion (M+ ) is weak or absent. The Cl mass spectrum provides reliable molecular mass information, but relatively little structural information (low abundance of the fragment ions). Compare with Fig. 11.4.

The effect of temperature on the relative molecular ion abundances in the mass spectra of CH4 and CD4 has been studied [493]. Isotope effects have been determined in the charge exchange mass spectra of partially deuterated methanes [682]. 

Light hydrocarbons, particularly methane, can diffuse relatively rapidly through imperfect seals. Because reservoir seals are usually not perfect the gases are likely to become depleted over geological time periods. Because diffusion is related to molecular mass, methane containing the 12C isotope diffuses more rapidly than that containing 13C, so there is an isotopic fractionation effect, with the residual gas becoming isotopically heavier (see Section 5.8.6b). 

Similarly, methane Cl spectrum of 18a was found to be dominated by the (C6H5C= CC6H5 + H)+ ion. A distinct molecular ion species at m/e value corresponding to (M + H)+ was observed in the methane mass spectra of this thiirene oxide (26% 2 40). Furthermore, the relative intensity of the (M +H)+ peak of 18a was shown to increase substantially in the isobutane and dimethyl amine Cl mass spectra91. 

The peak obtained in the spectrum is referred to as M+ peak and the intensity of this highest peak is called the base peak and it corresponds to the molecular weight of the substance and the intensities of all other peaks are expressed relative to the base peak. The base peak is caused by the fragment ion which is most stable and whose formation requires least energy. Mass spectrum of methane is given on page 268. 

The mass spectrum of methane ta relatively simple because few fraf mentations a re possible. As Figure 12.2a ahows, th4 base peak has miz 16. which corresponds to- the unbragmented methane cation radical, called the parent peak or the molecuJar ion mass spectrum also shows peaks at miz = 15 and 14, corresponding to cleavage of the molecular ion into CH, and CH2 fragments. 

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