Millimeters of mercury mm Hg

Because of the way in which gas pressure is measured, it is often expressed in millimeters of mercury (mm Hg). Thus we might say that the atmospheric pressure on a certain day is 772 mm Hg. This means that the pressure of the air is equal to that exerted by a column of mercury 772 mm (30.4 in) high. 

All these complications can be avoided when making field measurements by using the vacuum manometer shown in Fig. 6.6. The difference between the two mercury levels is the absolutely correct, inches of mercury absolute pressure, or millimeters of mercury (mm Hg). 

We have discussed the American system inches of mercury (in Hg) in Chap. 6, How Instruments Work. Of more immediate interest is Table 16.1. To do any sort of vacuum calculation, we need to convert to the absolute system, in millimeters of mercury (mm Hg). Unfortunately, we also need to correct measurements made with an American-type, (in Hg) vacuum gauge, for atmospheric pressure. You can interpolate between the two sets of data in Table 16.1, to correct for almost the entire range of typical atmospheric pressures. 

As is frequently the case with SI units, which must serve many scientific disciplines, the pascal is an inconvenient size for most chemical measurements. Thus, the alternative pressure units millimeter of mercury (mm Hg) and atmosphere (atm) are more frequently used. 

Oxygen is usually measured in terms of its partial pressure, p02, expressed as millimeters of mercury (mm Hg). p02 will, of course, be proportional to [02], and we may then rewrite Equation 7.2 as... 

Because gases are compressible, they exert pressure on their surroundings. Pressure is the force that is exerted over a unit area. For example, the atmosphere exerts a pressure known as atmospheric pressure. The Earth s atmosphere is a function of the location and the weather conditions, and it decreases with a higher altitude. The unit of pressure commonly used in chemistry is the atmosphere (atm). The standard atmosphere is 1 atm or a measurement of 760 millimeters of mercury (mm Hg or torr) on a manometer. 

Torr another name for millimeter of mercury (mm Hg). (5.1) Transfer RNA (tRNA) a small RNA fragment that finds specific amino acids and attaches them to the protein chain as dictated by the codons in mRNA. (22.6)... 

C thus, we often speak of 1 atm pressure as 760 mm or 30 inches (of mercury [Hg]). Because the density of mercury depends slightly on temperature, for accurate work it is necessary to specify the temperature and make the proper corrections to the density. A more precise term is the torr, defined as 1 torr = 1/760 atm (or 760 torr = 1 atm) at any temperature. Only at 0°C do the torr and the millimeters of mercury (mm Hg) coincide. These and other units of pressure are summarized in Table 9.2. 

Pressure is force per unit area and has dimensions of [M/LT2]. Pressure is an important measurement in many fields of science, and each held has different traditional units. The SI unit, the pascal [N/m2 or kg/(m sec2)], is commonly used, along with the dyn/cm2. Other pressure units frequently encountered include the millimeter of mercury (mm Hg), the atmosphere (atm), the bar (106 dyn/cm2), and the pound per square inch (psi). The origin of some of these units is implicit in their names the millimeter of mercury (also called a torr) is the amount of pressure that causes the mercury in a manometer to rise by 1 mm—an easy unit of measure for the laboratory experimentalist to use. Many of the common units of pressure and their interconversion factors are shown in Table A-7. 

The pressure (often expressed in millimeters of mercury, mm Hg) characteristic at any given temperature of a vapor in equilibrium with its liquid or solid form. 

Mercury barometers are simple and well known, so gas pressures are frequently expressed in terms of millimeters of mercury (mm Hg, or just mm). In recent years the unit torr has been used to indicate pressure it is defined as 1 torr = 1 mm Hg. 

Volatility is the tendency of a chemical to vaporize or give off fumes. The volatility of an agent varies with temperature and is expressed as the weight of vapor present in a given volume of air. Volatility is often confused with relative persistency. The volatility of a material (in ppm) can be calculated by multiplying its vapor pressure in millimeters of mercury (mm Hg) by 1315. 

The dispersion analyses that yielded the EPA tables were based on an averaging time of 6 s as compared with the 10 min and 60 min used for the toxic gas and vapor dispersion analyses. As with toxic materials, the neutral density equations would be used for listed gases and vapors that have a molecular weight <28 or where the product of vapor pressure and molecular weight is <500 millimeters of mercury (mm Hg). In contrast, the dense vapor equations would be used for listed... 

Millimeters of mercury (mm Hg) a imit of measurement for pressure, also called a torr 760 mm Hg = 760 torr = 101,325 Pa = 1 standard atmosphere. 

Boiling point is also related to vapor pressure and vapor content, although the relationship is opposite in nature. Materials with low boiling points and flash points will have high vapor pressure and high vapor content. The vapor pressure of a liquid is defined in the Condensed Chemical Dictionary as the characteristic at any given temperature of a vapor in equilibrium with its liquid or solid form. This pressure is often expressed in millimeters of mercury, mm Hg. ... 

Because instruments used for measuring pressure, such as the manometer (see Fig. 5.2), often use columns of mercury because of its high density, the most commonly used units for pressure are based on the height of the mercury column (in millimeters) the gas pressure can support. The unit millimeters of mercury (mm Hg) is called the torr in honor of Torricelli. A related unit for pressure is the standard atmosphere ... 

Standard atmospheric pressure, which corresponds to the typical pressure at sea level, is the pressure sufficient to support a column of mercury 760 mm high. In SI units this pressure is 1.01325 X 10 Pa. Standard atmospheric pressure defines some common non—SI units used to express gas pressure, such as the atmosphere (atm) and the millimeter of mercury (mm Hg). The latter unit is also called the torr, after Torricelli 1 torr = I mm Hg. Thus, we have... 

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