Density of a gas

Since one mole of an ideal gas occupies 22.4 L under standard temperature and pressure, we can readily calculate the density of an ideal gas under these conditions. Since density is mass/volume, and since the mass of one mole of a gas is simply its molar mass, the density of a gas is its molar mass divided by its molar volume  

The primary components of air are nitrogen (abont fonr-fifUis) and oxygen (abont one-fifUi). We will discuss the detailed composition of air in Section 5.6. 

Notice that the density of a gas is directly proportional to its molar mass. The greater the molar mass of a gas, the more dense the gas. For this reason, a gas with a molar mass lower than that of air tends to rise in air. For example, both heUum and hydrogen gas (molar masses of 4.00 and 2.01 g/mol, respectively) have molar masses that are lower than the average molar mass of air (approximately 28.8 g/mol). Therefore, a balloon filled with either helium or hydrogen gas floats in air. 

We can calculate the density of a gas more generally (under any conditions) by using the ideal gas law. To do so, we can arrange the ideal gas law as  

Since the left-hand side of this equation has units of moles/liter, it represents the molar density. We can obtain the density in grams/liter from the molar density by multiplying by the molar mass (M)  

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The pressure and the density of a gas are related by an equation of state. If the maximum pressure permitted within the centrifuge bowl is not too high, the equation of state for an ideal gas will suffice. The relationship between the pressure and density of an ideal gas is given by the weU-known equation ... 

Onee a gas or vapour has been mixed with air, it is the mean density of the mixture whieh is important (similar eonsiderations arise when mixing other gases). The mean density of a gas mixture is given by ... 

Density The measure of the amount of mass in a unit volume. The density of a gas is a function of its pressure and temperature, It can be determined by using the ideal gas laws. 

The density of a gas decreases when the temperature of the gas is increased. The heated air in the balloon is lighter than the air around the balloon. This density 5 difference causes the balloon to rise (after it is untethered). 

From this equation we see that the density of a gas is dependent on—... 

As the density of a gas increases, free rotation of the molecules is gradually transformed into rotational diffusion of the molecular orientation. After unfreezing , rotational motion in molecular crystals also transforms into rotational diffusion. Although a phenomenological description of rotational diffusion with the Debye theory [1] is universal, the gas-like and solid-like mechanisms are different in essence. In a dense gas the change of molecular orientation results from a sequence of short free rotations interrupted by collisions [2], In contrast, reorientation in solids results from jumps between various directions defined by a crystal structure, and in these orientational sites libration occurs during intervals between jumps. We consider these mechanisms to be competing models of molecular rotation in liquids. The only way to discriminate between them is to compare the theory with experiment, which is mainly spectroscopic. 

We see that, for a given pressure and temperature, the greater the molar mass of the gas, the greater its density. Equation 10 also shows that, at constant temperature, the density of a gas increases with pressure. When a gas is compressed, its density increases because the same number of molecules are confined in a smaller volume. Similarly, heating a gas that is free to expand at constant pressure increases the volume occupied by the gas and therefore reduces its density. The effect of temperature on density is the principle behind hot-air balloons the hot air inside the envelope of the balloon has a lower density than that of the surrounding cool air. Equation 10 is also the basis for using density measurements to determine the molar mass of a gas or vapor. 

The molar concentrations and densities of gases increase as they are compressed but decrease as they are heated. The density of a gas depends on its molar mass. 

Since few chemicals (e.g. hydrogen, methane, ammonia) have a molecular weight less than that of air, under ambient conditions most gases or vapours are heavier than air. For example, for common toxic gases refer to Table 3.1 for flammable vapours refer to Table 5.1. At constant pressure the density of a gas or vapour is, as shown, inversely proportional to the absolute temperature. As a result ... 

Whereas liquids and solids have well-defined densities, the density of a gas varies strongly with the conditions. To see this, we combine the ideal gas equation and the mole-mass relation and rearrange to obtain an equation for... 

Since the density of a gas is strongly dependent on the temperature and pressure, knowing only the volumetric flow rate is not adequate. For this reason the flow is given above as so many standard cubic feet per minute (scfm). This is the volume of fluid that would be transferred at a temperature of 60°F (15.6°C) and a pressure of 14.7 psia(1.033 kg/cm2). For design purposes, this rate is usually increased by 5%.27... 

The density of a gas depends quite strongly on its temperature, so hot air has a smaller density than does cold air colder air is more dense than hot air. From everyday experience, we know that something is dense if it tries to drop, which is why a stone drops to the bottom of a pond and a coin sinks to the bottom of a pan of water. This relative motion occurs because both the stone and the coin have higher densities than does water, so they drop. Similarly, we are more dense than air and will drop if we fall off a roof. 

We shall be precise regarding the way the various assumptions are made to derive our model. Let p(r, t) be the number density of a gas of stars, called later on the gravitational plasma following Prigogine and Seveme [5]. All the stars have the same mass m, and p is normalized in such a way that... 

However, the sohd density is approximately 1000 times the density of a gas at atmospheric pressure, and molecules in gases and liquids have much higher drSusivities than in solids. Therefore, the reacting boundary (I or R) moves very slowly compared to the motion of gas molecules to and from the boundary, and we can assume that concentration profiles near this boundary remain in steady state while we calculate the steady-state concentration profiles in the reaction... 

What happens to the density of a gas as it is compressed into a smaller volume ... 

Internal fluctuations are caused by the discrete nature of matter. The density of a gas fluctuates because the gas consists of molecules fluctuations in a chemical reaction arise because the reaction consists of individual reactive collisions current fluctuations exist because the current is made up of electrons radioactive decay fluctuates owing to the individuality of the nuclei. Incidentally, this explains why the formulas for fluctuations in physical systems always contain atomic constants, such as Avogadro s number, the mass of a molecule, or the charge of an electron. 

A gas may be defined as a homogeneous fluid of low density and low viscosity, which has neither independent shape nor Volume but expands to fill completely the vessel in which it is contained. The properties of gases differ considerably from the properties of liquids, mainly because the molecules in gases are much farther apart than molecules in liquids. For instance, a change in pressure has a much greater effect on the density of a gas than of a liquid. 

Since density is defined as the mass of gas per unit volume, an equation of state can be used to calculate the densities of a gas at various... 

VAPOR DENSITY. The density of a gas referred to the density of hydrogen or air as unity, If the density- of hydrogen is taken as 2, the vapor density is approximately the molecular weight if it is taken as one, the vapor density equals about half die molecular weight,... 

The molar concentration of a gas and a related quantity, the density of a gas, are both of great interest to meteorologists as well as to engineers. As we saw in Section G, the molar concentration is the number of moles of molecules divided by the volume of the sample (n/V). It follows from the ideal gas law that, for a gas behaving ideally,... 

As the density of a gas is increased and/or its temperature is lowered towards or below the critical temperature, Tc, new phenomena associated with the trapping and localization of positrons are sometimes encountered, indicating that many-body processes affect positron annihilation. We briefly describe these phenomena here, but a much more detailed treatment can be found in the review of Iakubov and Khrapak (1982). 

Note that the material density of a gas phase at a temperature of 300 K and pressure of 1 atm is given by... 

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