Behavior of Gases

Sxampub, Calculate the apparaat molecular w ht of a gas mixture con-aiating of three moles of methane, one mole of ethane and one mole of propane. 

The mole fractions of methane, ethane and propane in this mixture are 0.60, 0.20, and 0.20, respectively. Consequently 

The concept of apparent molecular weight is a very useful one since it permits the general gas law to be applied to gas mixtures, provided the molecular weight in the gas law is replaced by the apparent molecular weight. 

Dry air is a gas mixture consisting essentially of nitrogen, oxygen, and small amounts of other gases. Its cmnpoaition is ven in the table below 

Application of equation 11 leads to a value of 28,96 for the apparent molecular weight of air. However, for most engineering calculations a value of 29.0 is considered to be sufficiently accurate. 

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The ideal gas is a useful model of the behavior of gases and serves as a standard to which real gas behavior can be compared. This is formalized by the introduction of residual properties. Another useful model is the ideal solution, which sei ves as a standard to which real solution behavior can be compared. This is formalized by introduction of excess propei ties. 

The behavior of gases in air, as shown by studies in atmospheric physics and chemistry, depends on the physical and chemical properties of these gases, such as density and reactivity. Examples of gas and air densities are given in Table 13.20. 

Kinetic theory A mathematical explanation of the behavior of gases on the assumption that gases consist of molecules in ceaseless motion in space. The molecular kinetic energy depends on the temperature of the gas. 

In fluid mechanics the principles of conservation of mass, conservation of momentum, the first and second laws of thermodynamics, and empirically developed correlations are used to predict the behavior of gases and liquids at rest or in motion. The field is generally divided into fluid statics and fluid dynamics and further subdivided on the basis of compressibility. Liquids can usually be considered as incompressible, while gases are usually assumed to be compressible. 

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 ... 

As an example, we can explore the implications of our explanation of the behavior of gases. 

We have already seen that the behavior of gases is important to a chemist. The pressure-volume behavior leads to the particle model of a gas. Differences among gases (in properties such as color, odor, and solubility) show that the particles of one gas differ from the particles of another gas. In chemical reactions, the simple combining volume relationships support Avo-gadro s Hypothesis and, hence, give us a way to measure molecular weights. 

Regularities observed in the behavior of gases have contributed much to our understanding of the structure of matter. One of the most important regularities is Avogadro s Hypothesis Equal volumes of gases contain equal numbers of particles (at the same pressure and temperature). This relationship is valuable in the determination of molecular formulas—these formulas must be known before we can understand chemical bonding. 

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