Planning b
Apparatus and Materials:
A pressure gauge, the type used to measure tire pressure (a mechanical pressure
gauge) was used.
A gas syringe with plunger (glass if possible); eventually a 20 cm3 plastic syringe was
used.
Stand with clamp to hold pressure gauge in place.
Tough / rigid plastic tubing(not the soft rubber kind) to connect gauge and syringe
Source of nitrogen gas - an N2 tank that was securely attached to the working bench and
only operated by the teacher for safety reasons.
Stopcock grease.
Design for the control of variables:
1. The pressure gauge was attached to a tough plastic tubing for connection to the gas
syringe.
2. The plunger was greased very lightly, and then inserted into the barrel.
3. The plunger was completely inserted, connected to the nitrogen tank and 20.0 cm3
nitrogen gas was delivered into the syringe at the external pressure of 101.7 +- 0.5 kPa.
The stopcock was then closed. The syringe was then connected to the pressure gauge,
and the pressure recorded as 0.0 kPa, reflecting no additional pressure on the gauge
other than the pressure of the nitrogen gas at atmospheric pressure.
4. In order to check there was no leaking out of the syringe, the gas was compressed to
10.0 cm3, and it was found that the pressure reading remained the same for a oneminute
period.
5. Before any readings were taken, practice was obtained in moving the plunger in very
slowly in order to maintain constant temperature. Also, to make sure the needle of the
gauge did not stick, the gauge was gently tapped before a pressure reading was taken.
Design for the collection of sufficient relevant data:
1. To begin with, the 20.0 cm3 syringe was filled with 20.0 cm3 of nitrogen gas, the
stopcock closed, the syringe connected to the pressure gauge and the pressure
recorded to +/- 0.5 kPa.
2. The volume was then very gently decreased by 1.0 cm3 to 19.0 cm3 and allowed to
stand for a minute and the pressure recorded after gently tapping the mechanical gauge.
3. The procedure was then repeated for decrease in volume of 1.0 cm3 each until a total
of eleven readings were taken (considered sufficient to draw a graph).
4. In order to get duplicate readings, a second set of volume and pressure readings
taken, and the average values will be used in the processing of the data.
Data Collection: for nitrogen gas
Atmospheric pressure = 101.7 + / - 0.5 kPa
Note: To read the pressure gauge accurately, it was gently tapped and the pressure
needle reading was recorded to the nearest +/ - 0.5 kPa. Pressure readings were taken
twice each time starting with 20.0 cm3 gas. As the volume of gas was decreased, the
pressure increased. The quantitative data are presented in Table 1 below and the raw
data presented at end of write-up.
Table 1 : Pressure - Volume readings for Nitrogen gas at room temperature:
70.5
59.5
49.3
41.0
32.8
26.5
19.3
13.0
8.5
4.5
0.0
70.5
59.0
49.0
41.0
33.0
27.0
20.5
13.0
9.0
5.0
0.0
70.5
60.0
49.5
41.0
32.5
26.0
18.0
13.0
8.0
4.0
0.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
11
10
9
8
7
6
5
4
3
2
1
Average Pressure
P/ kPa *
Pressure , P2
+/ - 0.5 kPa
Pressure, P1
+/ - 0.5 kPa
Syringe Volume
+/ - 0.1 cm3
Reading
**Uncertainty in Pav is accounted for in Ptot - see data processing and presentation.
Data Processing and Presentation: The average pressure of the two pressure
readings is used, and the atmospheric pressure is added to each reading in order to
present the processed data.
5.81
6.21
6.62
6.99
7.46
7.81
8.26
8.70
9.09
9.43
9.81
172
161
151
143
134
128
121
115
110
106
102
70.5
59.5
49.3
41.0
32.8
26.5
19.3
13.0
8.5
4.5
0.0
101.7
101.7
101.7
101.7
101.7
101.7
101.7
101.7
101.7
101.7
101.7
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
11
10
9
8
7
6
5
4
3
2
1
Ptot-1/
10-3 x kPa-1
Total P = Atm P
+ P
+ / - 1 kPa
Average
pressure
P/kPa
Atmospheric
pressure
+/ - 0.5 kPa
volume of
syringe +/ - 0.1
cm3
reading
Graph 1 clearly shows that as volume decreases, the pressure increases. The straight
line Graph 2 of 1/P against V shows that 1/P is directly proportional to V or PV =constant.
To reflect the variables that were kept constant, this can be shown as (1/p V)n,T This is
Boyle`s Law statement as found in literature ( General Chemistry, Fourth Edition by
Whitten, Gailey and Davis. Saunders College Publ. 1992), namely the volume of a
fixed mass of gas at a constant temperature is inversely proportional to the pressure of
the gas. The law is explained following the hypothesis at the beginning of the write- up.
This investigation proves that PV=constant for nitrogen gas. In order to extend this
generalization to other gases, the experiments should be repeated with other gases
such as oxygen, carbon dioxide, and indeed to air - a mixture of gases.
Evaluation of Procedure and suggestions for improvement:
Soft rubber tubing was used first to connect the pressure gauge to the syringe, but it
expanded during the compression of the gas and thus changed the volume reading.
This was therefore replaced with a thicker plastic tubing with little possibility of expansion.
The aim was to use a glass rather than a plastic syringe to avoid the problem of
expansion but a glass syringe was not available, and one made of plastic was used.
This was fairly sturdy, and did not give rise to the problem of expansion under the
pressures that were used.
Since the plunger did not move smoothly through the barrel of the syringe, the plunger
was greased very lightly, and then inserted into the barrel to minimize friction.
In order to check that no gas was leaking out of the system, the gas was compressed to
10.0 cm3 and it was found that the pressure reading remained the same for a one-minute
period, so a closed system was present.
Before any readings were taken, practice was obtained in moving the plunger very
slowly in order to maintain constant temperature. Also, to make sure that the needle of
the gauge did not stick, the gauge was gently tapped before a pressure reading was
taken.
Data was duplicated by taking the pressure readings twice, each time by starting with
20.0 cm3 of nitrogen gas. After a second set of data was taken, the plunger did not
return to the 20.0 cm3 volume reading. In order to minimize the effect of any friction
between the plunger and the barrel of the syringe, it would have been better to take the
readings twice, once when the volume was decreased (gas compressed) and the
second time when the volume was expanded (gas expanded), and then taking the
average pressure value as the reading.
The volume occupied by the tubing and the connection to the gauge was not accounted
for. This volume should have been added to the volume of gas in the syringe to more
accurately reflect the total volume. Also, this volume could have been determined by
filling the connecting tube with water, and measuring the volume of water accurately using
a small measuring cylinder (this was in fact done after submitting the report, and the
volume was found to be 4.3 +/ - cm3 )