CHM 2046C Module 7II Chapter 13 Sample Exam Answers

Part V: Vapor Pressure Calculation 10 points

See Example 13.6 page 611 for a complete worked example. The following problem is exercise 13.7 on page 612. Also work problem #15 on pages 635.

If 0.50 g of pure water is sealed in an evacuated 5.0 L flask and the whole assembly is heated to 60^oC, will the pressure be equal to or less than the equilibrium vapor pressure at this temperature? (The equilibrium Vapor Pressure of water at 60 ^oC is 149.2 torre.)

1^st Calculate the equilibrium vapor pressure in atm:

149.2 torre		1 atm
--------------	X	-------- = 0.196 atm
1		760 torre

2nd Calculate the amount of water which will evaporate by using the ideal gas equation:

		PV (0.196 atm) (5.0 L)
n	=	-------- = -------------------------------------------- = 0.0359 moles
		RT (0.08205 L∙ atm/ K∙mole)(333 K)

3^rd convert 0.0359 moles of water to grams:

0.0359 mole		18.0 g
--------------	X	-------- = 0.6462 grams
1		1 mole

Answer: Putting 0.50 gram of pure water in the flask will result in all the water evaporating, because at that temperature 0.646 grams of water is the maximum mount of water that will vaporize at that temperature and .500g is less than the 0.646g.

If less what will be the pressure in the flask?

		nRT ( 0.0278 mole) (0.08205 L∙ atm/ K∙mole)(333 K)
P	=	-------- = -------------------------------------------------------- = 0.152 atm or
		V (5.0 L) 115.4 torre

What if you used 2.0 g of water?

0.646 grams will evaporate and 1.354 grams of water will remain a liquid.

Under either set of conditions, is any liquid water left in the flask, or does all the water evaporate? (The equilibrium vapor pressure of water at 60^oC is 149.4 torr). Answered above.