Honors Chemistry

Chapter 10-11 Worksheet #3 - Key

 

1.      Convert the following:

 

23o C = 296 K    4.54 atm = 458 kPa     19.3 kPa = 145 torr     195 K = -78 o C

 

2.      Explain the following: (assume any quantity not mentioned remains constant)

Increases – Molecules of gas don’t have as far to travel, so they hit more often.

 

Increase the outside pressure, or decrease the temperature.

 

Increases – molecules of gas move faster at a higher temperature, hit harder and more often

 

 

3.      What is diffusion and effusion?

Diffusion – When two gases are placed in the same container, the molecules of the two gases mix with each other.

 

Effusion – Gas molecules in a container with a small hole will move through the hole.

 

Both of these are explained by the constant, random motion of gas molecules, moving in a straight line, with lots of empty space around them

 

4.      Under what circumstances do we have to deal with partial pressure?  What do we mean by partial pressure? 

 

We deal with partial pressure when we have a mixture of gases.  Each gas exerts its own force based on the volume, temperature and number of gas molecules, independent of the other gas present

 

 

5.      A 250 ml glass flask is filled with carbon dioxide gas and sealed.  The pressure inside is 124 kPa.  If the temperature is raised from the current temperature (23.0o C) to 100.0o C, what does the pressure become?

 

P1/T1 = P2/T2     P2 = P1T2/T1 = (124 kPa)(373.2 K) / (296.2 K) = 156 kPa

 

 

6.      In an experiment, magnesium metal reacts with hydrochloric acid.  A gas measuring tube (also known as a eudiometer) captures the gas, and the volume of the gas is measured to be 37.6 ml.  If the room temperature is 21.4o C and the atmospheric pressure is 755 torr, how many moles of gas are in the tube. 

 

                 n = PV/RT =  (755 torr)(0.0376 l) / ( 62.4)(294.3) = .00154 moles                                                                                                                                                                                                                                                  

 

 

 

7.      In an experiment similar to problem 5 above, .0426 grams of magnesium react.  What volume of  hydrogen gas should be produced under the same conditions? (Write an equation first!)

 

            Mg   +   2 HCl    à    H2     +     MgCl2

 

             .0426 g Mg = .001753 moles Mg = .001753 moles H2   

 

            V = nRT/P = (.001753moles)(62.4)(294.6K)/(755torr) = .0427 l or 42.7 ml

 

8.      The following reaction consumes 348 ml of hydrogen sulfide gas (40.0o C and 365 kPa).  How many grams of the precipitate are produced?

                     2 Fe(OH)3(aq)     +     3 H2S à     2 Fe2S3(s)     +     3 H2O(l)

                                                               (molar mass = 207.892)

 

     n = PV/RT = (365 kPa)(0.348 l) / (8.32)(313.2 K)  = .04880 moles H2S

 

     .04880 moles H2S = .03254 moles Fe2S3 = 6.76 g Fe2S3

 

 

9.      A helium filled weather balloon is launched from its base where the temperature is 8.0oC and the pressure is 744 mm Hg.  As it rises in the atmosphere it expands.  At an altitude of 65,000 ft., the temperature has dropped to –27.0oC.  What must the pressure be at that attitude if the volume of the balloon has tripled?

 

         P1V1/T1 = P2V2/T2        or  P2 = P1V1T2/T1V2   since V2 = 3 V1  then P2 = P1V1T2/3V1T1

 

             Or  P2  = P1T2/3T1 = (744 mm Hg)(246.2 K) / 3(281.2 K) = 217 mm Hg

 

 

10.  If the volume of the helium in the balloon on the ground is 112,000 ft3 in problem 8 above, and 15 ft3 cylinders filled with helium to a pressure of 2200 lbs/in2 are used to fill it, how many of these cylinders were needed? (Hint: What’s changing as we fill the balloon?)

 

2200 lbs/in2 = 113740 mm Hg

 

P1V1 = P2V2

 

(113740 mm Hg)(15x) = (744 mm Hg)(112,000 ft3)   where x is the number of tanks

 

x = 48.8 or 49 tanks

 

11.  The gas pressure in a sealed glass flask is 1280 mm Hg.  The flask contains 0.346 moles of oxygen, 0.122 moles of nitrogen and 0.034 moles of helium.  What pressure is each gas producing in the flask?

 

.346 moles O2 + .122 moles N2  + .034 moles He = .503 moles of gas

 

      O2)  (0.346/0.502)(1280 mm Hg) = 882 mm Hg

      N2)  (0.122/0.502)(1280 mm Hg) = 311 mm Hg

      He)  (0.034/0.502)(1280 mm Hg) = 87 mm Hg