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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.
