Question

4. The Erlenmeyer flask used in the experiment (Charles Law: Determination of Absolute Zero) needed to be completely dry on the inside. If some water was initially present in the flask, how would it have affected your measurement of the volume of cooled air.

b. Why do you equalize the water level in the Erlenmeyer flask and the pan of water?

c. Use Charles Law to explain why it is not possible to reach temperatures below 0 K.

Answer 1

a) It is assumed that dry air completely fills up the Erlenmeyer flask in the experiment and hence, the volume of air is equal to the volume of the dry Erlenmeyer flask. If some water gets into the Erlenmeyer flask, then under the conditions of the experiment, the water inside the flask will vaporize and build up pressure (water vapor exerts pressure). The pressure inside the flask is therefore, greater than the pressure we started with. The main assumption of Charle’s law is that the pressure and the number of moles of dry gas must remain constant throughout the experiment and the presence of water inside the flask violates the assumption; therefore, Charle’s law will not hold good.

b) It is common knowledge that a difference in height between two liquids lead to an increase or decrease in pressure till the pressure in both the liquids are equalized. The basic assumption of Charle’s law is that the pressure must remain constant and hence, in order to do so, the water levels in the Erlenmeyer flask and the pan of water must be the same.

c) As per Charle’s law, the absolute zero of temperature is defined as the temperature where a sample of dry gas will occupy no volume whatsoever. When the absolute zero of temperature is reached, the gas occupies zero volume. It is not possible to reduce the volume of the gas any further (volume cannot be negative) and hence, we cannot reach temperatures below 0 K.