Q4. In NCERT’s experiment the box is first dried with anhydrous calcium chloride, which is removed before the water is put in. Why?
To remove the water vapour already in the air of the box. The pressure rise on the manometer then comes only from water evaporating out of the watch glass, starting from (almost) no vapour, so the steady value it reaches is the vapour pressure of water at that temperature.
Q5. Iodine crystals are sealed in a flask kept at a constant temperature. Once the violet colour of the vapour is steady, a few more crystals are added. Does the colour deepen?
No. Assuming iodine solid already remains, unchanged gas-space volume and optical path, and restoration of the same temperature, the equilibrium vapour pressure does not depend on the added solid amount, so the vapour, and its colour, stay the same; the extra crystals just add to the solid.
Q6. Why does an opened bottle of soda go flat faster on a hot day than in a refrigerator?
For CO2 in water near these temperatures, solubility decreases as temperature rises (NCERT, page 171). In the warm drink less CO2 can stay dissolved at a given pressure, so more tends to leave. The escape rate also depends on mass transfer, mixing and bubble nucleation; equilibrium solubility alone does not determine how fast it leaves.
Q7. At 25 °C liquid A has a vapour pressure of 60 kPa and liquid B one of 20 kPa. Which is more volatile, and which has the lower normal boiling point?
A: a higher vapour pressure means the liquid evaporates more readily. The normal boiling-point order cannot be determined from these two pressures at one temperature alone; it requires their vapour-pressure curves up to 1 atm.
Q8. At a certain temperature 0.040 g of oxygen dissolves in 1 L of water under 1.0 atm of pure oxygen. How much dissolves from air, in which the partial pressure of oxygen is 0.21 atm?
By Henry’s law the mass dissolved is proportional to the partial pressure of the gas: 0.040 × 0.21, about 0.0084 g, that is, about 8.4 mg per litre.