Measurements on Gases

To completely describe the state of a gaseous substance, its volume, amount, temperature, and pressure are specified. The first three of these quantities were discussed in earher chapters and will be reviewed briefly in this section. Pressure, a somewhat more abstract quantity, will be examined in more detail. 

A gas expands uniformly to fiU any container in which it is placed. This means that the volume of a gas is the volume of its container. Volumes of gases can be expressed in liters, cubic centimeters, or cubic meters  

Most commonly, the amount of matter in a gaseous sample is expressed in terms of the number of moles (n). In some cases, the mass in grams is given instead. These two quantities are related through the molar mass, MM  

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The temperature of a gas is ordinarily measured using a thermometer marked in degrees Celsius. However, as we will see in Section 5.2, in any calculation involving the physical behavior of gases, temperatures must be expressed on the Kelvin scale. To convert between °C and K, use the relation introduced in Chapter 1  

Typically, in gas law calculations, temperatures are expressed only to the nearest degree. In that case, the Kelvin temperature can be found by simply adding 273 to the Celsius temperature. 

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Having detemiined A b and knowing that the gas constant R = 8.314JK from macroscopic measurements on gases, determine Avogadro s number L from the relationship... 

The specific conditions used in the calculation—1 atm pressure and 0°C (273.15 K)—are said to represent standard temperature and pressure, abbreviated STP. These standard conditions are generally used when reporting measurements on gases. Note that the standard temperature for gas measurements (0°C, or 273.15 K) is different from that usually assumed for thermodynamic measurements (25°C, or 298.15 K Section 8.6). Note also that the standard pressure for gas measurements, still listed here and in most other books as 1 atm (101,325 Pa), has been redefined to be 1 bar (100,000 Pa). Thus, the new standard pressure is 0.986 923 atm, making the standard molar volume 22.711 L rather than 22.414 L. 

Plots of / versus T (°Q for several gases. The solid lines represent experimental measurements on gases. The dashed lines represent extrapolation of the data into regions where these gases would become liquids or solids. Note that the samples of the various gases contain different numbers of moles. 

During the following discussion we shall employ wherever possible dipole moment data obtained from measurements on gases, since these values are the most accurate. When such data are not available the values are taken from measurements on dilute solutions in non-polar solvents. Such values... 

Measurements on gases require much longer path-lengths than those for condensed phases 10 cm is usual for compounds at relatively high concentrations. For trace concentrations multiple reflection cells provide pathlengths of several metres in compact designs. Detection limits are well below 1 ppm for many compounds. 

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