September 2015

# A different way to do gas laws — Part 1: Pressure-volume relationship

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John Eix (retired), Oakville ON

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This three-part series will describe how to use syringes, pressure gauges and mason jars to study gas laws — Boyle’s law, Gay-Lussac’s law and Avogadro’s law. Most likely you did not learn the properties of gases using mason jars, syringes and pressure gauges. The advantages of this equipment are safety and environmental impact, increased ease of use and reduced time to get good, meaningful data.

In this Boyle’s law lab, a student fills a 60-mL syringe with the gas to be studied. Students attach a pressure gauge to the syringe (Fig. 1). Slowly push the syringe’s piston into the barrel recording pressure readings at 3-mL intervals until a volume of 30 mL is reached. The compress pressure readings are shown in the data set below. The piston of the syringe is then slowly pulled out of the barrel. Again the pressure is recorded at 3-mL readings. This is the expanded pressure reading.

A gas distribution apparatus (Fig. 2) is used to distribute gases to the students. The apparatus consists of a plastic bag, rubber stopper and stopcock that can be connected via plastic tubing to a gas generator or cylinder. The bag is first emptied. Then it is filled with the desired gas and the stopcock is closed. The gas-filled bag can be used to give students gas by having students connect their syringe to the stopcock and slowly pulling out the piston to fill the syringe while the bag deflates.

Fig. 1 Pressure gauge attached to syringe.

A typical set of student data using nitrogen (Fig. 3) is shown below along with a graph of the data (Fig. 4). In the data workup, the “pressure compress” and the “pressure expand” are averaged and added to the atmospheric pressure to give “pressure total”. Note: The volume of the gauge and associated connectors has been measured at 4.3 ± 0.3 mL. With this syringe and pressure gauge apparatus, the pressure-volume relationship can be found for various gases. Appropriate gases are nitrogen, oxygen, carbon dioxide, methane and propane. The apparatus (Fig. 1) is called EQ 877 Boyle’s law apparatus and is available at S17 Science for US$23 US and CAN$27.60. Kits with additional equipment to do other gas investigations can also be purchased. See www.s17science.com.

Fig. 2 Gas distribution apparatus.

Fig. 3 Sample data collected by students for nitrogen gas.
Reading | Vol Gauge (± 0.3 mL) | Vol (± 0.5 mL) | Vol Total (±-0.8 mL) | Press. Compress (± 0.5 kPa) | Press. Expand ( ± 0.5 kPa) | Press Total (± 1 kPa) | 1/V (L-1) | Uncert (L-1) | PV (L*kPa) | uncert (L*kPa)
0 | 4.3 | 60.0 | 64.3 | 0 | 0.0 | 101 | 15.6 | 0.2 | 6.5 | 0.1
1 | 4.3 | 57.0 | 61.3 | 4.0 | 3.0 | 105 | 16.3 | 0.2 | 6.4 | 0.1
2 | 4.3 | 54.0 | 58.3 | 9.0 | 9.0 | 110 | 17.2 | 0.2 | 6.4 | 0.1
3 | 4.3 | 51.0 | 55.3 | 14.0 | 14.0 | 115 | 18.1 | 0.3 | 6.4 | 0.1
4 | 4.3 | 48.0 | 52.3 | 23.0 | 22.0 | 124 | 19.1 | 0.3 | 6.5 | 0.2
5 | 4.3 | 45.0 | 49.3 | 31.0 | 29.0 | 131 | 20.3 | 0.3 | 6.5 | 0.2
6 | 4.3 | 42.0 | 46.3 | 38.0 | 38.0 | 139 | 21.6 | 0.4 | 6.4 | 0.2
7 | 4.3 | 39.0 | 43.3 | 49.0 | 47.0 | 149 | 23.1 | 0.4 | 6.5 | 0.2
8 | 4.3 | 36.0 | 40.3 | 60.0 | 59.0 | 161 | 24.8 | 0.5 | 6.5 | 0.2
9 | 4.3 | 33.0 | 37.3 | 72.0 | 71.0 | 173 | 26.8 | 0.6 | 6.5 | 0.2
10 | 4.3 | 30.0 | 34.3 | 74 | 72.5 | 174 | 29.2 | 0.7 | 6.0 | 0.2

Atmospheric Pressure: 758.5 ± 0.2 mm (101.1 ± 0.5 kPa)

Average PV (L*kPa) and uncert (L*kPa), respectively: 6.4 0.2

Standard deviation for Uncert (L-1): 0.14

Fig. 4 Graph of student data.

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