Gases atomic weight measurement

The elements whose isotopes are routinely measured with gas inlet mass spectrometers are carbon (12C and 13C, but not 14C), oxygen (160, 170, l80), hydrogen ( H, 2H, but not 3H), nitrogen (14N and 1SN) and sulphur (32S, 33S, 34). Stable isotopes of H, C, N, O, and S occur naturally throughout atmosphere, hydrosphere, lithosphere, and biosphere. They are atoms of the same elements with a different mass. Each element has a dominant light isotope with the nominal atomic weight (I2C, 160,14N, 32S, and H) and one or two heavy isotopes (l3C, nO, 180, 15N, 33S, 34S, and, 2H) with a natural abundance of a few percent or less Table 1). 

However, it should not be necessary to make any heat capacity measurements at all, or any assumptions as to the thermal properties of the solid and liquid states in order to calculate the correct value for the entropy of hydrogen. Since hydrogen gas at low temperatures consists entirely of molecules in the zero rotational state, its entropy will be that of a monatomic gas of atomic weight 2.016. The entropy at 298°K. will be obtained by adding the integral f CPd In T over the proper temperature range. The heat capacity may be separated into a constant term 5/2R and the rotational term Cr. 

Atomic-Weight Determinations by the Gas-Density Method. If a sufficiently careful measurement of the density of a gas is made, under conditions such that the gas obeys the perfect-gas law, a good value for the molecular weight of the gas can be obtained, which can be used to find the atomic weight of one of the elements in the gas. The way to determine this ideal value of the density of a gas is to determine the density of the gas at smaller and smaller pressures, and to extrapolate to zero pressure-all gases approach the perfect-gas law in their behavior as the pressure becomes very low. This method of atomic-weight determination has been used extensively in recent vears by the Spanish cheijaist E. Moles. 

The calorimetric measurement of A H by direct combination of the elements has been the subject of numerous investigations (jL ) O Hare et al. ( ) performed six combustion experiments on a sample of rhombic sulfur which contained 60 ppm oxygen as the major impurity. The sulfur ( 0.5 gram) was burned in 5-6 atmospheres of fluorine which had a purity of 99.97%. Spectrometric and gas chromatographic analyses of the product gases indicated that the sole combustion product was SFg. We adjust their result AjH (298.15 k) = -271.7+0.24 kcal mol to correspond to an atomic weight of sulfur equal to 32.06, and we obtain A H (SFg, g, 298.15 k) = -291.7g 0.2 kcal mol . The rounded value of -291.7 0.2 kcal mol is adopted in this tabulation. Less extensive measurements (3 determinations) by Schroder and Sieben (2) on a portion of the same sample used by O Hare et al. (J[) gave AjH (SFg, g, 298.15 K) = -291.4 kcal mol which provides confirmation for our selected value. 

White, Friedman, and Johnston (343) have measured the critical constants for normal hydrogen and have found 33.244 K. and 12.797 atmospheres. Woolley, Scott, and Brickwedde have presented data on the dissociation energy and the thermodynamic properties for the ideal diatomic gas, including contributions from nuclear spin. We have omitted the spin entropy in compiling our tables. Thermodynamic properties for the ideal monatomic gas have been computed at the National Bureau of Standards (395). Note that the reference state represents 2 gram atomic weights for this element. 

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