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To Demonstrate Boyle's Law by Syringe Experiment

25th Dec 2019 @ 4 min read

Physical Chemistry

Boyle's law is a pressure versus volume relationship. The law was discovered by Robert Boyle in the 17th century. It states the pressure of a fixed amount of a gas is inversely proportional to its volume at a constant temperature. The law can be empirically proven. The article discusses an experimental method to verify the law using a syringe.

Experiment: Sealed syringe

The experiment is very simple. It can be performed at home. When the tip of a syringe is sealed with a cap, the air inside the syringe is isolated from the atmosphere. This will fix the amount of the gas. The weights (books) are added upon the plunger of the syringe. It will push the plunger downwards; in other words, the air in the syringe is compressed. By recording the weights of the books added and the volume reading from the syringe, we can establish the pressure-volume relationship.

Objective

To verify Boyle's law and to plot the pressure-volume graph

Materials

  1. A 140 mL disposable syringe
  2. A seal cap
  3. Two wooden blocks: one with the central hole on which the syringe will be mounted and the other which will be attached to the plunger
  4. Books that can comfortably place on the wooden block
  5. A lubricant
  6. A wooden split or tongue depressor
Boyle's law syringe experiment
Experimental diagram

Nomenclature

  1. Vi is the volume reading.
  2. wi is the weight on each book.
  3. w0 is the initial weight, which is the sum of the weight of the wooden piece resting on the plunger and the weight of the plunger.
  4. Wi is the total weight on the air inside the syringe.

Procedure

  1. Take the syringe and paste a thin layer of the lubricant to the rubber gasket of it with the help of a wooden split or tongue depressor. This will reduce friction.
  2. Pull the plunger of the syringe upwards—around 110 mL.
  3. Now, attach the seal cap to the syringe.
  4. When a small amount of downward force is applied to the plunger, it should revert to the original position. If not, the more lubrication is necessary or the seal cap is not properly attached.
  5. Mount the tip of the syringe to the cavity of the wooden block and place it in the upside-down position as shown in the above figure.
  6. Fix the other block to the plunger of the syringe such that the syringe is perpendicular to the blocks.
  7. Measure the initial volume reading.
  8. Place a book on the wooden piece and record the volume reading.
  9. Repeat the previous step for two books, three books, four books, and five books.
  10. Remove all the books and weigh each. Also, weigh the wooden block with the plunger; it will give w0.
  11. Reset the apparatus. Repeat all the above steps twice. Take the average of all three sets.

Precautions

  1. The proper lubrication is necessary to eliminate friction.
  2. The end of the syringe should tightly fix by a sealed cap. Otherwise, the experiment will fail.
  3. The syringe must be properly fixed, so it can firmly withstand the weights.

Observation

The initial weight (w0) is 92 g.

The total weight is Total weight.

The observation table is as follows:

Observation table
No. of booksVolume reading in mL (Vi)Average (Vi)Weight in g (wi)Total weight in g
Set 1Set 2Set 3
0102100104102092
160586262505597
2505644505031100
3323834345031603
4263232304992102
5242826265012603

Calculation

The pressure on the air inside the syringe is the pressure exerted by the weights plus atmospheric pressure.

Total pressure on the syringe

The pressure exerted by the weights is the force exerted by the weights divided the inner area of the syringe.

Now, Force (Fw) is mass (Wi) times acceleration (a).

Pressure exerted by the weights

Here, r is the inner radius of the syringe, which can be measured; r = 0.005 m. a is the acceleration due to gravity; a = 9.81 m s−2.

For Wi = 92 g,

Pressure exerted by 92 g

Assume atmospheric pressure (Patm) as 101.325 kPa.

Total pressure when the weight is 92 g

Similarly, we can calculate the total pressure for the rest.

The calculation table is as follows:

Calculation table
No. of booksPw in kPaPi in kPaVi in mLPiVi
011.5112.810211500
174.6175.96213100
2137.4238.75011900
3200.2301.53410200
4262.5363.83010900
5325.1426.42611100

We have to plot the graph of Pi vs Vi and PiVi vs Vi.

Results

The Pressure vs volume graph is as follows:

Boyle's law experimental graph
Pressure vs volume

The pressure-volume vs volume graph is as follows:

Boyle's law experimental graph pressure-volume
Pressure-volume vs volume

Conclusion

The PV curve from the above figure is satisfactory. As the pressure of the air increases, its volume decreases. The air obeys Boyle's law. Also, the product of pressure and volume approximately constant and its value is independent of volume or pressure.

Also, check a laboratory method: To verify Boyle's law»

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Boyle Law Gas Law Boyle Law Experiment

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