Oxygen, pressure-volume properties

Unlike solids and liquids, the density of a gas depends very strongly on the temperature and pressure. Also, unlike solids and liquids, we can easily calculate the density of a gas if we know the temperature and pressure. For example, what is the density of air under normal conditions (25°C and 750 torr) Recall that air is approximately 79% nitrogen and 21% oxygen (by volume). If we want to calculate the density of a sample, we need to know its mass and volume. Since density is an intrinsic physical property, we can take any sample size we want, so let s take a sample volume of 1.0 L. 

Many gases are colorless substances we take for granted, They continually surround us and supply us with needed oxygen and supply plants with needed carbon dioxide. Gases unique property of compressibility allows for quick and observable changes. Changes in pressure, volume, temperature, and other physical properties can be calculated with various ideal gas laws. 

For geologic purposes, the dependence of the equilibrium constant K on temperature is the most important property (4). In principle, isotope fractionation factors for isotope exchange reactions are also slightly pressure-dependent because isotopic substitution makes a minute change in the molar volume of solids and liquids. Experimental studies up to 20kbar by Clayton et al. (1975) have shown that the pressure dependence for oxygen is, however, less than the limit of analytical detection. Thus, as far as it is known today, the pressure dependence seems with the exception of hydrogen to be of no importance for crustal and upper mantle environments (but see Polyakov and Kharlashina 1994). 

Activity Measurements. To test catalytic properties of various samples partial oxidation of methanol to formaldehyde was studied in a flow micro-reactor operating under normal atmospheric pressure (10). For each run about 0.2 g of catalyst sample was used and the activities were measured at 173 C in the absence of any diffusional effects. The feed gas consisted of 72, 2 and by volume of nitrogen, oxygen and methanol vapor respectively. Reaction products were analysed with a 10% Carbowax 20 M column (2m long) maintained at 60 C oven temperature. 

All observable quantities of the macroscopic property of a thermodynamic system, such as the volume V, the pressure p, the temperature T, and the mass m of the system, are called variables of the state, or thermodynamic variables. In a state of the system all observable variables have their specified values. In principle, once a certain number of variables of the state are specified, all the other variables can be derived from the specified variables. The state of a pure oxygen gas, for example, is determined if we specify two freely chosen variables such as temperature and pressure. 

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