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D-3
To calculate P
wc
, recall that at 25
C, a column of water 13.57 mm in height exerts the same pressure as
1 mm column of mercury (that is, the ratio of the respective densities is 13.57). Therefore, the relationship
between P
wc
and the height of the water column (h) is:
P
wc
= [
1 mm Hg
13.57 mm H2O
] x h
(mm)
= 0.0737
1 mm Hg
mm H2O
x h
(mm)
To determine P
wc
in mm Hg, calculate the product of the height of the water column (h
(mm)
) in millimetres
and 0.0737.
The Ideal Gas Law may be rearranged into the form of a straight line graph:
V =
nRT
P
A graph of volume of gas (y-axis) versus moles of gas (x-axis) will give a straight line for which the
slope(m) is equal to (RT/P) and the y-intercept will be zero. If the pressure and temperature of the gas are
known, then “R” can be calculated from the slope of the line. An experimental value of “R” will also be
determined using this graphical method. This value will be compared to the average value of “R” found
from the individual runs.
EXPERIMENTAL METHOD
The experimental procedure involves reacting sulfamic acid, the limiting reagent, with an excess of NaNO2.
A known volume of a sulfamic acid solution at a known concentration will be used in each run. Excess
NaNO2 will be used to ensure the reaction goes to completion.
There is an error that will affect the value of “R” that is calculated. This error is due to the heat of reaction.
As the sulfamic acid reacts with the sodium nitrite, the heat released causes the temperature in the
Erlenmeyer flask to increase. Let's assume that the total volume of the Erlenmeyer flask is 300 mL. You
will have added 19 mL of sulfamic acid and 10 mL of sodium nitrite to the flask, so that the volume
available for the gas, V1, is (385 - 23) = 262 mL. If the temperature of the contents of the Erlenmeyer flask
increases during the reaction, the air and gases within the Erlenmeyer flask will expand. Since the glass
walls of the flask do not move noticeably for small changes in temperature, the volume available to the air-
gas mixture is constant.
If the flask was sealed, the pressure inside would increase. However, in this experiment the flask is not
sealed. The extra volume of the air-gas mixture escapes into the buret from the Erlenmeyer flask (pressure
remains constant). Hence, the volume of gas collected in the buret will be greater than that expected from
the amount of sulfamic acid used in the run. To estimate the error in the measured volume of nitrogen in
the buret, proceed as follows.
