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To Verify Boyle's Law Experimentally

29th Oct 2019 @ 4 min read

Physical Chemistry

Boyle's law is a famous gas law studied in physics and chemistry. It relates pressure and volume of gas keeping other parameters (amount of gas and temperature) constant. There are several ways to verify the law. The article covers a standard laboratory method to verify the law by studying the relation between pressure and volume.

Experiment: Air column

Objective

To verify Boyle's law i.e. for a given amount of gas, absolute pressure is inversely proportional to volume at a constant temperature.

Apparatus

Air column with measuring scale, Bourdon pressure gauge, oil reservoir, hand air pump, barometer

The air column is connected to the oil reservoir such that the air in the column is completely sealed off by the oil.

Boyle's law experiment
Experimental diagram for the verification of Boyle's law

Nomenclature

  1. Vi is the volume reading of the gas in the air column.
  2. Pi is the pressure reading in the pressure gauge.

Procedure

  1. Connect the apparatus as shown in the above diagram.
  2. The connection of the oil reservoir to the air column must be such that there is no leakage and the air is completely sealed off by the oil.
  3. The hand air pump is attached to the oil reservoir.
  4. After assembling all the types of equipment, open the air tap and start pumping air through the air pump.
  5. Keep pumping the air until the oil is reached in the upper part of the column. As the air flows in the oil reservoir, the pressure inside the system increases. This can be observed in the pressure gauge.
  6. Close the air tap once the oil no longer rises and the pressure gauge reading is constant (i.e. reached its peak).
  7. Wait for two to three minutes to cool down the compressed air and note the pressure reading and the volume reading.
  8. Now, slightly open and then quickly closed the air tap to let some air escape the system. This will lower the oil in the column.
  9. After two to three minutes, note the pressure and volume reading.
  10. Repeat the above two steps such that seven to eight observations are recorded.
  11. Prepare the observation table and follow the calculations as shown below.
  12. Plot pressure vs volume and pressure vs inverse volume graph
  13. Finally, conclude the experiment based on the graphs.

Precaution

  1. The air in the column should be completely sealed off by the oil.
  2. The air pump is tightly connected to the oil reservoir.
  3. Each reading must be taken after two to three minutes of closing the air tap.
  4. The volume readings must be taken by levelling the eyes to the meniscus of the oil.

Observation

A sample observation table is provided below.

Sample observation table
Pressure reading (kPa)Volume reading (cm3 or ml)
65025
45033
29048
23057
19564
16072
11089

Calculation

Pressure and volume are the two parameters are measured throughout the experiment. The experiment is performed at a constant temperature which is room temperature. The amount of the gas in the column is also constant. Thus, the conditions of Boyle's law are satisfied.

The volume readings are in cm3 and the pressure readings are in kPa. The pressure measured is gauge pressure, not absolute. So, we need to add the atmospheric pressure to the readings to obtain absolute pressure. The atmospheric pressure is obtained from a barometer, but it is considered as 1 atm i.e. 101.325 kPa.

For example, a gauge pressure of 650 kPa is an absolute pressure of 650 + 101.325 = 751.325 kPa. The table below lists absolute pressure.

Gauge pressure to abosolute pressure
Pressure reading (kPa)Absolute pressure (kPa)
650751.325
450551.325
290391.325
230331.325
195296.325
160261.325
110211.325

The final table consists of absolute pressure (kPa), volume (cm3), inverse volume (cm−3), and pressure-volume (kPa cm3).

Calculation table
Absolute pressure (kPa)Volume (cm3)Inverse volume (cm−3)Pressure-volume (kPa cm−3)
751.325250.04019 000
551.325330.03018 000
391.325480.02119 000
331.325570.01719 000
296.325640.01619 000
261.325720.01419 000
211.325890.01119 000
Note: † rounded off to two significant figures.

As observed from the table above, pressure-volume nearly remains constant i.e. PV = k.

Result

The graph of pressure vs volume and pressure vs inverse volume are plotted.

Boyle's law experiment graph (pressure vs volume)
As pressure decreases, volume increases exponentially. The graph is an exponential curve.

As expected, the above graph (pressure vs volume) is an exponential curve. As observed from the graph, pressure decreases with rise in volume or vice versa.

Boyle's law experiment graph (pressure vs inverse volume)
The graph of pressure vs inverse volume is a straight line passing through the origin.

The graph of pressure to inverse volume is a straight line and follows the equation y = mx.

Conclusion

The experiment is successfully studied, and the system obeys Boyle's law. From the graphs, it can be concluded that the pressure of the gas in the column is inversely proportional to the volume of the gas at a constant temperature for a fixed amount of gas.

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