The vapour pressure of water at 300 K in a closed container is 0.4 atm. If the volume of the container is doubled, its vapour pressure at 300 K will be:

0.4 atm

The correct answer is option 3. 0.4 atm.

The vapor pressure of a substance is the pressure exerted by its vapor when it is in equilibrium with its liquid or solid phase at a given temperature. It is a characteristic property of the substance and depends primarily on the temperature.

According to the ideal gas law, the pressure and volume of a gas are inversely proportional at a constant temperature. However, it's important to note that the ideal gas law is not directly applicable to liquids.

In the given scenario, the vapor pressure of water at 300 K is stated as 0.4 atm. This means that at that temperature, the equilibrium between the liquid and vapor phases of water is established, and the vapor pressure of water molecules escaping from the liquid is equal to 0.4 atm.

Now, if the volume of the container holding the water is doubled, it does not have any direct impact on the vapor pressure of the water. The vapor pressure of a substance is determined by the temperature and the properties of the substance itself, rather than the volume of the container.

Therefore, in this case, as the temperature remains constant at 300 K, the vapor pressure of water will remain the same regardless of any changes in the volume of the container. Thus, the correct answer is that the vapor pressure of water will remain at 0.4 atm.