226 Aptitude Test Problems in Physics
become m = psatS/a, and the vessel will contain
a liquid having the mass
M Psa tS
a2.20. Boiling of water is the process of intense
formation of steam bubbles. The bubbles contain
saturated water vapour and can be formed when
the pressure of saturated water vapour becomes
equal to the atmospheric pressure (760 mmHg, or
105 Pa). It is known that this condition is fulfilled
at a temperature equal to the boiling point of
water: Tbo n = 100 °C (or 373 K). By hypothesis,
the pressure of saturated water vapour on the
planet is Pe, = 760 mmHg, and hence the temper-
ature on the planet is T = Thal] = 373 K. Using
the equation of state for an ideal gasPolu
P RTbotiwhere p, is the molar mass of water, and po is the
atmospheric pressure, and substituting the nu-
merical values, we obtain p = 0.58 kg/m 3.
2.21. When we exhale air in cold weather, it is
abruptly cooled. It is well known that the saturated
vapour pressure drops upon cooling. Water vapour
contained in the exhaled air becomes saturated
as a result of cooling and condenses into tiny water
drops ("fog").
If we open the door of a warm but on a chilly
day, cold air penetrating into the but cools water
vapour contained in the air of the hut. It also
becomes saturated, and we see "fog", viz. the drops
of condensed water.
2.22*. It is easier to solve the problem graphically.
The total pressure p in the vessel is the sum of the
saturated water vapour pressure Psat and the
pressure of hydrogen pH.. According to the equa-