11
Gases
In order for chemists to prepare and handle gases under a variety of conditions,
they must understand the relationships between the weight, volume, temperature, and pressure of a gas sample. Measurement of the first three of these
quantities is relatively straightforward. Because the measurement of pressure
can present some complications, we discuss it before we consider the interrelationship of all four variables.
MEASUREMENT OF GAS PRESSURE
The most common laboratory instrument used to measure gas pressure is a
manometer, a glass U-tube partially filled with a liquid (Figure 11-1). Mercury is
the most commonly used liquid, because it is fairly nonvolatile, chemically
inactive, and dense, and it does not dissolve gases or wet (adhere to) glass.
Because mercury does not wet glass, its meniscus will curve upward instead of
down, and its position is recorded as that of the horizontal plane tangent to the
top of the meniscus.
Figure 11-1 illustrates the measurement of gas pressure with a manometer. In
part (a), both sides of the mercury column are at the same pressure, and the
mercury level is the same in both tubes. In part (b), the righthand tube has been
evacuated and, as a result, the mercury has risen until the weight of the mer-
Gases
In order for chemists to prepare and handle gases under a variety of conditions,
they must understand the relationships between the weight, volume, temperature, and pressure of a gas sample. Measurement of the first three of these
quantities is relatively straightforward. Because the measurement of pressure
can present some complications, we discuss it before we consider the interrelationship of all four variables.
MEASUREMENT OF GAS PRESSURE
The most common laboratory instrument used to measure gas pressure is a
manometer, a glass U-tube partially filled with a liquid (Figure 11-1). Mercury is
the most commonly used liquid, because it is fairly nonvolatile, chemically
inactive, and dense, and it does not dissolve gases or wet (adhere to) glass.
Because mercury does not wet glass, its meniscus will curve upward instead of
down, and its position is recorded as that of the horizontal plane tangent to the
top of the meniscus.
Figure 11-1 illustrates the measurement of gas pressure with a manometer. In
part (a), both sides of the mercury column are at the same pressure, and the
mercury level is the same in both tubes. In part (b), the righthand tube has been
evacuated and, as a result, the mercury has risen until the weight of the mer-