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Chapter 7
A barometer is a glass tube, which is closed at one end and open at the other, that is filled
with mercury and immersed with the open end down into a pool of mercury. Only part of
the mercury drains from the tube, and the resulting column has a height that depends
upon the atmospheric pressure. The column, which is supported by the molecules in the
atmosphere, has a height of 760 mm at sea level at normal conditions. Gas pressures can
be determined by measuring the height of the mercury column that they support. The
height is usually given in mm, and the pressure is given in mm of Hg or torr (1 torr = 1
mm Hg). The following are common ways of expressing the normal pressure at sea level:
•
14.7 lb/in
2
•
1 atm
•
760 mm Hg = 760 torr
•
1.01x10
5
Pa, where Pa is the Pascal, the SI unit of pressure. 1Pa = 1 kg·m
1
·s
2
The pressure of a gas in the laboratory is often found as the difference in heights of two
columns of a liquid in a manometer.
1.
P
gas
= P
bar
+ h if mercury level is higher on atmosphere side
2.
P
gas
= P
bar
 h if mercury level is higher on gas side
Boyle's Law and Pressure
The volume of a given amount of gas at a constant temperature varies with its pressure
PV = k(n,T) (Pressure*Volume=constant which is the amount of gas times the
temperature)
The relationship between pressure and volume is known as Boyle's Law. The constant,
k(n,T) is a function of the number of moles of gas (n) and its temperature (T). Boyle
found that the product of the pressure of a gas and its volume is constant as long as the
number of moles of the gas and the temperature of the gas are held constant. The
following equation is often used for Boyle's Law. It indicates that the PV product is
unchanged in processes that involve no change in temperature or amount of gas.
PressureVolume Product
Equation 7.2
P
f
V
f
= P
i
V
i
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View Full Document where P
f
V
f
is the final PV product and P
i
V
i
is the initial product.
The volume of a given amount of gas increases linearly with temperature if the pressure
is unchanged.
V = k(n,P)T + A
where the constant k(n,P), a function of the number of moles and pressure of the gas, is
the slope of the line and A is the intercept.
The absolute or Kelvin temperature scale is related to the Celsius scale as K =
o
C +
273.15
The absolute temperature must be used in all equations involving the temperature of a
chemical system. However, either the Celsius or absolute scales can be used in equations
involving a
change
in the temperature of a system because the unit size is the same in
both scales
7.18. Charles' Law
V = kT, where k is a constant that depends on the amount and pressure of the gas, and T
is the temperature in kelvins
The volume of a fixed amount of gas at constant pressure is proportional to its
temperature expressed in kelvins
Charles' Law
Equation 7.3
V =
k(n,P)
T
This equation is known as Charles' Law. Charles' Law indicates that the V/T ratio of a
gas is constant so long as n and P are unchanged. The following is another way of
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This note was uploaded on 04/21/2008 for the course CH 101 taught by Professor Bigham during the Spring '08 term at N.C. State.
 Spring '08
 BIGHAM

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