Practical Investigation 2

Boyle’s Law

1.1 Aim:

To investigate the relationship between pressure and the volume of an enclosed gas at a constant temperature

1.2 Hypothesis:

The greater the pressure of gas, the smaller (less) the volume of the gas.

2.3.1 What is the independent variable?

Pressure

2.3.2 What is the dependent variable?

Volume

3.1 Use the table to record the results. Also indicate thecorrect SI unit in each column.

Pressure of gas (kPa)

Volume of gas (cm3)

𝟑 pV

80 43 0,023 3 440

90 37 0,027 3 330

100 33 0,030 3 300

110 30 0,033 3 300

120 27 0,037 3 240

130 24 0,042 3 120

140 22 0,045 3 080

3.2.1 Pressure versus volume

10 20 500

50

100

150

Pre

ssur

e (k

Pa

)

Volume (cm3)

Graph of pressure versus volume

200

250

30 40

3.3 Now draw a graph of Pressure vs 1/Volume.

0,01 0,02 0,0501/volume (cm-3)

Pre

ssur

e

(kP

a)Graph of pressure versus 1 / volume

0,03 0,04

50

100

150

200

250

4. Conclusion

A graph of p versus V produces a hyperbolic curve, characterised by an inverse proportionality. Therefore, p is possibly inversely proportional to V.This is confirmed by drawing a graph ofp versus 1 / .It produces a straight line through the origin, so p α 1 / , provided T is constant; so p = / , provided T is constant;so pV = k.

4. Conclusion

The values in the table (column 4) also confirm that this product (pV) is a constant. The gradient of the straight line is given by:

p ÷ 1 / = pV = k ≡ Pa × = N / × = N.m = J

The product pV is therefore an indication of the internal energy of the gas particles. The temperature remained constant throughout, so the temperature was related to the internal kinetic energy, which also remained constant. The constant, k, is dependent on temperature.

At very high pressures, where the volume of the gas particles themselves have to be taken into account, i.e. if the gas in the apparatus is not completely dry, so that the water vapour liquefies, or if the gas in the apparatus itself liquefies, the graph will show the following deviation:

0,01 0,02 0,0501/volume (cm-3)

Pre

ssu

re (

kPa)

Graph of pressure versus 1 / V

0,03 0,04

50

100

150

200

250

If the experiment is carried out at a higher temperature, the gradient of the graph will also be larger since the internal kinetic energy of the molecules are now also higher and the gradient, or k, related to the energy will also be larger.

5.1 The shape of graph 1 of pressure versus volume is a hyperbolic curve and thus the pressure is possibly inversely proportional to volume.

5.2 The shape of graph 2 of pressure versus the reciprocal of volume is a straight line through the origin. The pressure is thus inversely proportional to the volume of an enclosed gas.

5.3 Define Boyle's law…5.3.1 in words

The volume of an enclosed gas is inversely proportional to the pressure of the gas provided the temperature of the gas is constant.

5.3 Define Boyle's law…5.3.2 in symbols

p

5.4.1 Is the gradient of the graph p versus 1 / V constant?

Yes

5.4.2 What quantity was kept constant during this experiment?

Temperature

5.4.3 Determine by cancellation of units what the dimension (unit) of the gradient is.

pV = k thusPa m3 = N/m2 m3 = N.m = J.

5.4.4 What does the gradient ofgraph 2 represent?

Energy (Temperature)

5.5 Briefly explain what is meantby an “ideal gas”.

5.5.1 Particles are identical5.5.2 Particles have no volume5.5.3 Gas has a volume only due to

motion of the particles5.5.4 No attractive or repulsive forces

between particles 5.5.5 Collisions are completely elastic

5.6 The product pV (remains constant / increases / decreases) when investigating the relationship between pressure and volume of an ideal gas.

5.7 Identify TWO causes of possible deviations from the ideal gasstate that occur with non-ideal gases under high pressureand / or low temperature.

At high pressure an actual gas will liquefy.At low temperatures, an actual gas will condense (liquefy).

5.8.2 Which of the gases N2, CO2, H2 and He should showthe most ideal behaviour?

He – small atom and weak forces between atomsH2 – small volume and weak intermolecular forces

5.10 Is the gas closest to ideal at high or low temperatures?

High temperature

5.11 Why is it necessary to allow at least 3 minutes betweenreadings?

So that the temperature of the enclosed gas can return to room temperature each time.

5.16 Why should the temperature be measured in Kelvin ?

The pressure of a gas is not zero at zero oC, but theoretically at 0 K.

5.17 What is meant by STP?

(273 K) 0 oC and 101,3 kPa