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D-1
EXPERIMENT D:
DETERMINATION OF
THE
UNIVERSAL GAS CONSTANT
OBJECTIVE
Gases can be described in terms of the following four parameters: pressure (P), volume (V), temperature
(T), and the moles of gas present (n). In this experiment you will study the relationship that exists between
these variables.
THEORETICAL CONSIDERATIONS
Experimental investigations of the relationships between the temperature, pressure, volume, and moles of
gases is expressed in the Ideal Gas Law :
P V = n R T
If T is the temperature in Kelvin or absolute temperature, P is the pressure of the gas in atmospheres, V is
the volume in liters, n is the amount of substance in moles, then R, a constant known as the universal gas
constant, has a value of 0.08206 L
atm
K
-1
mol
-1
.
In this experiment, you will perform a chemical reaction that will evolve a predetermined number of moles
of nitrogen gas, n
N2
, The N
2(g)
produced will be collected in a buret over water. You will also determine its
pressure (P
N2
), volume (V) and temperature (T). The value of R can be obtained by substituting these
values into the ideal gas law.
Nitrogen gas is produced by the reaction between sulfamic acid, HSO3NH2
,
and sodium nitrite, NaNO2
,
as
shown in equation (1):
NaNO2
(aq)
+ HSO3NH2
(aq)
NaHSO
4
(aq)
+ N2
(g)
+ H2O
(l)
(1)
One mole of nitrogen gas is produced for each mole of NaNO2 and HSO3NH2 which reacts. The apparatus
to be used is shown in Fig. 1. The reaction takes place in the Erlenmeyer flask where the nitrogen gas is
produced. This gas will be collected in the buret by the displacement of water.
Since the nitrogen gas formed will be collected over water, the vapor pressure of water must also be taken
into account. The total pressure is the sum of the pressure of the nitrogen gas collected and the vapor
pressure of water. The vapor pressure of water as a function of temperature is given in the table of water
vapor pressure on the Chemistry Data Sheet. To determine the exact vapor pressure of water for
temperatures not shown in the table, for example 22.3°C, you must use linear interpolation between the
nearest given values, in this case at 22°C and 23°C.
