Gases molar mass, determination

The attachment of halide ions to anilines may therefore afford abundant adduct ions which are valuable for selective detection and molar mass determination of the analytes. For example, Cl- attachment has proven suitable for the gas chromatography/CI-MS monitoring of human exposure to several aromatic amines112. 

A rough value of the molar mass is usually sufficient to determine the molecular formula of a substance. For example, if chemical analysis of a gas yields an empirical formula (CH2) , then the molar mass must be some multiple of 14 g/mol the possibilities are 28,42, 56,70, and so on. If a molar mass determination using Eq. (2.20) yields a value of 54 g/mol, then we may conclude that n = 4 and that the material is one of the butenes. The fact that the gas is not strictly ideal does not hinder us in this conclusion at all. In this example the possible values of M are well enough separated so that even if the ideal gas law were wrong by 5 %, we would still have no difficulty in assigning the correct molecular formula to the gas. In this example it is unlikely that the ideal gas law would be in error by as much as 2 % for a convenient choice of experimental conditions. 

Among the various mass spectrometry techniques, MALDI is probably the most important as it provides an absolute method for molar mass determination and molar mass distribution, as well as information on end groups and copolymer composition. The MALDI process consists of the ablation of the polymer molecules dispersed in a matrix typically made up of aromatic organic acids. The matrix needs to be able to absorb at the wavelength of a laser (usually 337 nm). This process excites the matrix molecules, which vaporize at the same time, the polymer molecules desorb into the gas phase, where they are ionized. Thus, the role of the matrix is that of transferring the laser energy to the polymer molecnles. 

The ideal gas law offers a simple approach to the experimental determination of the molar mass of a gas. Indeed, this approach can be applied to volatile liquids like acetone (Example 5.4). All you need to know is the mass of a sample confined to a container of fixed volume at a particular temperature and pressure. 

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. 

I 4 Determine molar mass from gas density and vice versa (Example 4.6). 

The ideal gas equation and the molecular view of gases lead to several useful applications. We have already described how to cany out calculations involving P-V-n-T relationships. In this section, we examine the use of the gas equation to determine molar masses, gas density, and rates of gas movement. 

We can use the ideal gas equation to calculate the molar mass. Then we can use the molar mass to identify the correct molecular formula among a group of possible candidates, knowing that the products must contain the same elements as the reactants. The problem involves a chemical reaction, so we must make a connection between the gas measurements and the chemistry that takes place. Because the reactants and one product are known, we can write a partial equation that describes the chemical reaction CaC2(. ) +H2 0(/) Gas -I- OH" ((2 q) In any chemical reaction, atoms must be conserved, so the gas molecules can contain only H, O, C, and/or Ca atoms. To determine the chemical formula of the gas, we must find the combination of these elements that gives the observed molar mass. 

Use the gas data to determine the molar mass. The problem gives the following data about the unknown... 

Relating tnofecular propoFiies lo gas properties Worklrsg witli vapor pressure Using gas properties to determine molar mass... 

B The gas s molar mass is its mass (1.27 g) divided by its amount in moles. The amount can be determined from the ideal gas equation. 

We can use Graham s law to determine the rate of effusion of an unknown gas knowing the rate of a known one or we can use it to determine the molecular mass of an unknown gas. For example, suppose you wanted to find the molar mass of an unknown gas. You measure its rate of effusion versus a known gas, H2. The rate of hydrogen effusion was 3.728 mL/s, while the rate of the unknown gas was 1.000 mL/s. The molar mass of H2 is 2.016 g/mol. Substituting into the Graham s law equation gives ... 

In this equation, u is the osmotic pressure in atmospheres, n is the number of moles of solute, R is the ideal gas constant (0.0821 Latm/K mol), T is the Kelvin temperature, V is the volume of the solution and i is the van t Hoff factor. If one knows the moles of solute and the volume in liters, n/V may be replaced by the molarity, M. It is possible to calculate the molar mass of a solute from osmotic pressure measurements. This is especially useful in the determination of the molar mass of large molecules such as proteins. 

E—This problem depends on the ideal gas equation PV = nRT. R, V, and T are known, and by using the partial pressure for a gas, the number of moles of that gas may be determined. To convert from moles to mass, the molar mass of the gas is needed. 

The molar mass (molecular mass) of a volatile substance is determined in this experiment. The mass of a sample of vapor is initially determined. This mass, along with the volume of the container, the pressure, and the temperature, is used with the ideal gas equation to... 

The ideal gas law is a powerful tool that the chemist—and now you—can use to determine the molar mass of an unknown gas. By measuring the temperature, pressure, volume, and mass of a gas sample, you can calculate the molar mass of the gas. 

Instruments with indirect pressure measurement. In this case, the pressure is determined as a function of a pressure-dependent (or more accurately, density-dependent) property (thermal conductivity, ionization probability, electrical conductivity) of the gas. These properties are dependent on the molar mass as well as on the pressure. The pressure reading of the measuring instrument depends on the type of gas. 

Hydrogen, H2. The question states that the ratio of the rates is 4.0. Oxygen gas is a diatomic element, so it s written as O2 and has a molar mass of 32.00 g/mol. Substitute these known values into Graham s law to determine the molar mass of the unknown gas. The problem states that the unknown gas effuses at a rate 4.0 times faster than oxygen, so put the unknown gas over oxygen for the ratio. (In short, the unknown gas is A, and the oxygen is B). 

After you have the molar mass, you can check the periodic table to determine the identity of the element by matching up the molar mass. No element specifically matches the 2.00 g/mol however, if you remember that hydrogen is diatomic and is always written as H2, you can identify the unknown gas as hydrogen ... 

A gas component A in air is absorbed into water at latm and 20 °C. The Henry s law constant of A for this system is 1.67 X 10 Pa m kmol h The liquid film mass transfer coefficient and gas film coefficient I(q are 2.50x10 and 3.00 X10" ms respectively, (i) Determine the overall coefficient of gas-liquid mass transfer (ms ). (ii) When the bulk concentrations of A in the gas phase and liquid phase are 1.013 X 10 Pa and 2.00 kmol m , respectively, calculate the molar flux of A. 

In an experiment to determine the molar mass of a newly synthesized chlo-rofluorocarbon (CFC) gas for use in a refrigeration system, it was found that. 25 mL of the gas effused through a porous barrier in 65 s. The same volume... 

RTIM)112. These values have a fixed relation to each other. Place these three quantities in order of increasing magnitude. Show that the relative magnitudes are independent of the molar mass of the gas. Using the smallest speed as the reference point, determine the ratio of the larger values to the smallest. 

Ealy, Jr., "Determining the Molecular Weight of a Gas," Chemical Demonstrations, A Source-book for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), pp. 19-20. The molar mass of butane is determined by the vapor density method. 

The empirical formula CH2 is not a stable substance. It is necessary to determine the molar mass to determine the molecular formula. If this hydrocarbon were a gas or an easily volatilized liquid, its molar mass could be determined from the density of the gas, as shown in Chapter 5. Supposing such a determination yields a molar mass of about 55 g/mol, what is the molecular formula ... 

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