19. On a winter day, the air temperature is 2158C, and the humidity is 0.001 kg/m3. (a) What is the relative humidity? (b) When this air is brought inside a building, it is heated to 20C. If the humidity isn't changed, what is the relative humidity inside the building?

21. Inside a building, the temperature is 20C, and the relative humidity is 40 percent. How much water vapor is there in each cubic meter of air?

23. An apartment has the dimensions 10 m by 5 m by 3 m. The temperature is 25C, and the relative humidity is 60 percent. What is the total mass of water vapor in the air in the apartment?

25. The temperature of the air in thermals decreases about 10C for each 1,000 m they rise. If a thermal leaves the ground with a temperature of 30C and a relative humidity of 31 percent, at what altitude will the air become saturated and the water vapor begin to condense to form a cloud? (In other words, at what altitude does the temperature equal the dew point?)

27. What is the Carnot efficiency of a heat engine operating between the temperatures of 300C (573 K) and 100C (373 K)?

29. As a gasoline engine is running, an amount of gasoline containing 15,000 J of chemical potential energy is burned in 1 s. During that second, the engine does 3,000 J of work. (a) What is the engine's efficiency? (b) The burning gasoline has a temperature of about 4,000F (2,500 K). The waste heat from the engine flows into air at about 80F (300 K). What is the Carnot efficiency of a heat engine operating between these two temperatures?

31. An irreversible process takes place by which the entropy of the universe increases by 1.67 J/K. If the temperature of the environment in which the process occurred is 220 K, how much energy was made unavailable for useful work as a result?