Question about vapor pressure

Question

A flask containing nothing other than air at 100.0 kPa is filled partway with water and then sealed and left to sit at 20C. The total pressure of a mixture of gases is equal to the sum of the pressures of each gas in the mixture.

a. What will happen to the pressure inside the flask as time goes on? Explain, in terms of vapor pressure.

b. What is the vapor pressure of water at 20C?

c. Based on your answer to a and b and the given information, what would the new pressure be in the flask once vapor-liquid equilibrium is reached?

Answer 1

a). The pressure inside the flask will increase as time passes. The reason for this is that some water molecules will go from liquid to vapor (evaporation) and this will add pressure (vapor pressure of water).

b). The vapor pressure of water at 20ºC is 2.339 kPa (you find this value by looking it up in a book or table)

c). New pressure after water has equilibrated = 100.0 kPa + 2.330 kPa = 102.330 kPa = 102.3 kPa