Torr, unit of pressure

Torr Unit of pressure equal to one part in 760 of a standard atmosphere, also equal to the pressure at the bottom of a column of mercury 1 millimeter high with vacuum above it. 

Throughout this chapter the torr. millibar, and pascal are used as units of pressure, according to the original data rather then converting the first two to the SI unit. Note than 1 Pa = 0.01 mbar = 0.0076... 

Before we start describing the gas law relationships, we will need to describe the concept of pressure. When we use the word pressure with respect to gases, we may be referring to the pressure of a gas inside a container or we might be referring to atmospheric pressure, the pressure due to the weight of the atmosphere above us. The pressure at sea level is 1 atmosphere (atm). Commonly, the unit torr is used for pressure, where 1 torr = 1 mm Hg (millimeters of mercury), so that atmospheric pressure at sea level equals 760 torr. The SI unit of pressure is the pascal (Pa), so that latm = 760 mm Hg = 760 torr = 1.01325 X 10s Pa (or 101.325 kPa). 

Concentrations of gaseous pollutants are often expressed in terms of parts per million (ppm) by volume, and time is expressed in minutes. Use of these concentration units must be reflected in the units used for the rate constants as well for example, second-order rate constants are in units of ppnt-1 min-1. Occasionally, gas concentrations are given in units of mol L-1 or in units of pressure such as Torr, atmospheres, or Pascals these can be converted to the more conventional units... 

The pascal represents a very small pressure, and therefore the most common applications, such as tire pressure, will use kilopascals (kPa) instead of Pa. Other units of pressure include the torr (or millimeter of mercury, mmHg), inches of mercury, the atmosphere (atm), and the bar. A torr is an amount of pressure necessary to support a column of mercury 1 mm in height. One atmosphere of pressure is loosely defined by the atmospheric pressure at sea level, but is more precisely defined as the pressure necessary to support a column of mercury 760 mm in height. One bar of pressure is equal to 100 kPa. The relationships between the various units of pressure are given below ... 

Volume may be expressed in a variety of units—liter (L), milliliter (mL), cubic meter (m3), cubic centimeter (cc or cm3). Similarly, pressure can be expressed in a variety of units, but the standard unit of pressure is the atmosphere (atm) one atmosphere (1 atm) is defined as the pressure needed to support a column of mercury 760 mm in height at 0°C at sea level. In honor of Evangelista Torricelli, the Italian inventor of the barometer, the unit torr is used and is equal to 1 mm Hg. Thus... 

A variety of pressure units are in use. These units are related to the column of mercury shown in Figure 6.2. The height of the mercury column in the tube is commonly measured in inches or in millimeters. As you listen to the weather report on the evening news, notice that the atmospheric pressure is given in inches. Where the metric system is in use, millimeters are favored. Millimeters are synonymous with torr, a unit named for the Italian scientist Evangalista Torricelli. Less frequently used units are atmospheres and pascals. The following equation shows the relationship among these various units of pressure. 

Units of pressure and vacuum should be identified as force per unit of area, and pressure units typically are mbar, psi, and kg/cm2. Pressures below 1 torr (= 1 O 3 atm) for many years were described in relationship to standard atmosphere in various ways such as ... 

For gases, this force can be a factor of their motion or their weight. Atmospheric pressure is caused by the weight of air particles that are attracted toward earth. Inside a sealed container, pressure is exerted by the collisions of particles on the sides of the container. By determining the force of those collisions on a given area, you can determine the pressure exerted by the particles. There are many units that describe pressure. The SI unit of pressure is the pascal, Pa. The kilopascal is a bit more practical as a unit, however, since a pascal is quite small. Other units of pressure include millimeters of Hg (mm Hg), torr, bar, and atmospheres. The relationship between the units is as follows ... 

You might have noticed that torr and mm Hg have the same value. On the AP test, units of pressure are usually expressed as atm in the ideal gas problems (R uses atm as the pressure unit) and mm Hg in partial pressure problems. 

P) force per unit area. The SI unit of pressure is the pascal, defined as one newton per square meter. Other common pressure units are the atmosphere, the bar, and the Torr. 

The term ract/t/m refers to the condition of an enclosed space that is devoid of all gases or other material content. It is not experimentally feasible to achieve a perfect vacuum, although one can approach this condition quite closely. It is possible routinely to obtain a vacuum of 10 Torr and with more sophisticated techniques 10 ° Torr (1.3 X 10 ° bar) it is even possible by special techniques to obtain a vacuum of 10 ° Torr, or about 30 molecules per cubic centimeter. One Torr, the conventional unit of pressure in vacuum work, is the pressure equivalent of a manometer reading of 1 mm of liquid mercury ... 

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. 

At sea level, the atmosphere keeps the mercury in a barometer at an average height of 760 mm, which is 1 atmosphere. One millimeter of mercury is also called a torr, after Evangelista Torricelli, the Italian physicist who invented the barometer. Other units of pressure are listed in Table 1. 

Units of pressure The SI unit of pressure is the pascal (Pa). It is named for Blaise Pascal, a French mathematician and philosopher. The pascal is derived from the SI unit of force, the newton (N), which is derived from three SI base units the kilogram, the meter, and the second. One pascal is equal to a force of one newton per square meter 1 Pa = 1 N/m. Many fields of science still use more traditional units of pressure. For example, engineers often report pressure as pounds per square inch (psi). The pressures measured by barometers and manometers can be reported in milhmeters of mercury (mm Hg). There also is a unit called the torr, which is named to honor Torricelli. One torr is equal to one mm Hg. 

At sea level, the average air pressure is 760 mm Hg when the temperature is 0°C. Air pressure often is reported in a unit called an atmosphere (atm). One atmosphere is equal to 760 mm Hg or 760 torr or 101.3 kilopascals (kPa). Table 13-1 compares different units of pressure. Because the units 1 atm, 760 mm Hg, and 760 torr are defined units, they have as many significant figures as needed when used in calculations. Do the problem-solving LAB to see how the combined pressure of air and water affects divers. 

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 atmospheric pressure is measured with an instrument called a barometer, which is also called a closed manometer. When you watch your local weather report, you will often hear the meteorologist discuss the barometric pressure, which is a measure of the force exerted by the atmospheric gases per unit of area. The news often reports pressure in inches of mercury, because a barometer, as are many thermometers, is filled with mercury. The SI unit for pressure is the Pascal, which is derived from Newtons and meters. Other units that are commonly used to measure pressure include the atmosphere (atm), torr, and millimeters of mercury (mm of Hg). You should make sure that you know how to convert between the various units of pressure. The necessary conversion factors are shown here. 

Torr A unit of pressure the pressure that will support a column of mercury 1 mm high at 0°C. 

Although pressure is formally defined as force unit area, it is more commonly expressed in milli-meters of mercury, mmHg, or atmospheres, atm. The mmHg is also called the torr in honor of Torricelli, who built the first barometer in 1643. The SI unit of pressure is the Pascal, although it is not widely used in the United States. One atmosphere is 1.013 x 105 Pascal (Pa) or 101.3 kilopascal (kPa). The relationships between the units of pressure are... 

Note that the relationships between 1 atm, 760 mmHg, and 760 torr are exact. You should be comfortable converting from one unit of pressure to another. 

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