CHEMISTRY
LAB REPORT: EXPERIMENTAL PROOF OF BOYLE’S LAW
OCTOBER 3, 2012
LAB PERFORMED BY:
Ankon Rahman
Introduction Boyle’s law is an established law of chemistry given by Robert Boyle (1627-1691). This law of chemistry is one of the gas laws of chemistry. This law describes the relationship between the pressure and the volume of a gas when the temperature remains constant. Boyle’s law states that if the temperature is kept constant, volume of a gas in inversely proportional to the pressure applied to it. For example let a pressure of P1 is applied to a gas which makes its volume V1. At this condition, again a pressure of say P2 is applied to the gas which compresses it to a volume V2. According to Boyle’s law, P1.V1 = P2.V2 . So, if the product of the applied pressure and resultant volume is constant, we can say that it supports Boyle’s law.
Equipments A syringe, weights (each of 1kg), two wooden rectangular bodies supporting the syringe.
Diagram
Figure 1: Experimental proof of Boyle’s law.
Procedure 1. At first, the two wooden blocks are placed on both ends of the syringe. 2. The initial volume of the syringe is set to 35 cm3. 3. A 1 kg weight is placed on the syringe, which creates pressure and decreases the volume. The volume changes, which is recorded. 4. In the same way, another 1 kg weight is placed on the syringe. The changed volume is recorded. 5. Procedure 3 and 4 are repeated and a total weight upto 5 kg is placed on the syringe. The change of volume for increase of each kg is recorded.
Data Collection and Calculation Raw data: Raw data table 1:
Syringe Area/m 0.00001 m Atmospheric pressure/Pa 1 Pa Initial Volume of syringe/ cm3 1 cm3 2
2
Initial Data 2.8*10-4 101200 Pa 35
The syringe area has been pre calculated. The current atmospheric pressure is collected from weather forecast report. The smallest unit of the scale of the syringe is 1mL = 1 cm3. As it is an analog instrument, therefore, the uncertainty = ½ = 0.5 cm. But as the measurement is done in two places (one at the top of the syringe and another at the bottom), so absolute error = 0.5 cm3 *2 = 1 cm3.
Raw data table 2: Total mass/kg 0 1 2 3 4 5
Volume/cm3 1 cm3 35 29.5 25 21 18.5 16.5
Processed Data: We know the masses of the respective weights. But in order to find out the pressure, the total force must be known. The total force is the product of the masses of the weights and the acceleration due to gravity. This is derived from Newton’s laws of motion which states F = mg where F is the total force, m is the mass, and g is the value of acceleration due to gravity. The unit of the total force is kgms-2 which can be said as Newton (N). Processed data table 1: Total force(mg)/N 0 9.8 19.6 29.4 39.2 49
Volume/cm3 35 29.5 25 21 18.5 16.5
Now as the total force is known, therefore, the pressure on the syringe can be calculated. We know that pressure is the force per unit volume, that is, P = F/A where P is pressure, F is the force and A is the area. The F is derived from processed data table 1 and A(2.8*10-4 m2) is derived from Raw data table 2. The unit of pressure is kgm-1s-2 or Pascal (Pa). Processed data table 2: Pressure/Pa 0 35000 70000 105000 140000 175000
Volume/cm3 35 29.5 25 21 18.5 16.5
Besides the pressure on the syringe created by the weights, the atmospheric pressure is also acting on the syringe. Therefore total pressure = pressure applied by the weights + atmospheric pressure (atmospheric pressure = 101.2kPa = 101200 Pa)
Processed data table 3: Volume/cm3 35 29.5 25 21 18.5 16.5
Total pressure/Pa 101200 136200 171200 206200 241200 276200
The values in Processed Data Table 4 are plotted in a graph. 40 35
Volume/cm^3
30 25 20 15 10 5 0 0
50000
100000
150000
200000
250000
300000
Pressure/Pa Graph 1: Volume/pressure curve.
Calculation: If the temperature is kept constant, volume of a gas in inversely proportional to the pressure applied to it. For example let a pressure of P1 is applied to a gas which makes its volume V1. At this condition, again a pressure of say P2 is applied to the gas which compresses it to a volume V2. According to Boyle’s law, P1.V1 = P2.V2 . So, if the product of the applied pressure and resultant volume is constant, we can say that it supports Boyle’s law.
The product of the total pressure and the volume P.V for each trial is given below: P1.V1 = 35*101200 = 3542000 P2.V2 = 29.5*136200 = 4017900 P3.V3 = 25*171200 = 4280000 P4.V4 = 21*206200 = 4330200 P5.V5 = 18.5*241200 = 4462200 P6.V6 = 16.5*27600 = 4557300 We can say that all the products are approximately equal to 4*106.
Conclusion and Evaluation From the values received through the experiment, it can be said that the experiment was accurate and precise. But due to a few reasons, the accuracy and precision have decreased to some extent. However, it can be said that Boyle’s law – ‘If the temperature is kept constant, volume of a gas in inversely proportional to the pressure applied to it’ is valid for this experiment. The product of the total pressure and volume is more or less the same for each trial which is approximately equal to 4*106. Other than this, if we analyze the graph, we can see that the line joining the points is almost linear. If it was completely linear, that would mean that the product of P and V are the same. As the line was not completely linear and at the same time, the product of pressure and volume were not same for all the trials, it is clear that the experiment was partly inaccurate and imprecise. There are a few reasons for this. At the end of the experiment, when the weights were removed from the top of the syringe, it was seen that the volume of the region inside the syringe has not returned to its initial value (35 cm3), but a value less than that. This means that air has escaped from the syringe somehow, that is the syringe was not airtight. Besides this, though the written mass of each weight was given 1kg, but the mass may have changed because of corrosion, rust, etc. These factors may have affected the original value of the experiment which has caused it to be inaccurate and imprecise to some extent. To avoid these errors in the future, it should be assured that the syringe that is taken for the experiment is airtight. And the weights which are taken for the experiment should be weighed properly and the exact value of their masses should be recorded.
