Question

Two ideal gas thermometers A and B use oxygen and hydrogen respectively. The following observations are made:

| Temperature | Pressure thermometer A | Pressure thermometer B |
| Triple point of water Normal melting point of sulphur | 1.250 x 105 Pa1.797 x 105 Pa | 0.200 x 105 Pa0.287 x 105 Pa |

(a) What is the absolute temperature of normal melting point of sulphur as read by the thermometer A and B?

(b) What do you think is the reason for the slightly different answers from A and B? (Thermometers are not faulty). What further procedure is needed in the experiment to reduce the discrepancy between the two reading?

Answer 1

(a) For thermometer A,

Ptr = 1.250 x 105 Pa

Ttr = 273.16 K

p = 1.797 x 105

and T = ?

Now, using Ptr/Ttr = p/T ⇒ T = pTtr/Ptr

= {1.797 x 105 x 273.16}/{1.250 x 105} = 392.69 K

For the thermometer B,

Ptr = 0.200 x 105 Pa, Ttr = 273.16 K

p = 0.287 x 105 Pa, T = ?

Ttr = p.Ttr/Ptr = {0.287 x 105}/{0.200 x 105} x 273.16 = 391.98 K

(b) The discrepancy between the two reading is due to the fact that the gases are not perfectly ideal gases. To reduce the discrepancy, the readings should be taken at low pressures so that the gases could show perfect behaviour.