Cliffs AP Chemistry, 3rd Edition

Part B: Question 7

7. Restatement: Explain how MW measured just above boiling point deviates from its ideal
   value in terms of the ideal gas law.
   The ideal gas equation, PV = nRT, stems from three relationships known to be true for
   gases:
   i) The volume is directly proportional to the number of moles: V ~ n
   ii) The volume is directly proportional to the absolute temperature: V ~ T
   iii) The volume is inversely proportional to the pressure: V ~ 1 / P
   n, the symbol used for the moles of gas, can be obtained by dividing the mass of the gas
   by the molecular weight. In effect, n= mass / molecular weight (n= m / MW).
   Substituting this relationship into the ideal gas law gives
   PV=mRTMW::

Solving this equation for the molecular weight yields

MW=mRTPV::

Real gas behavior deviates from the values obtained using the ideal gas equation because

the ideal equation assumes that (1) the molecules do not occupy space and (2) there is no

attractive force between the individual molecules. However, at low temperatures (just

above the boiling point of the liquid), these two postulates are not true and one must use

an alternative equation known as the van der Waals equation, which accounts for these

factors.

Because the attraction between the molecules becomes more significant at the lower tem-

peratures, the compressibility of the gas is increased. This causes the product P ⋅V to be

smaller than predicted. PV is found in the denominator in the equation listed above, so the

molecular weight tends to be higher than its ideal value.

Part B: Question 8

Question 8 could be answered by using the bullet format. You should try to arrange your points
in logical order, but because time is a consideration, you may not be able to organize all of
your bullets in perfect sequence.

8. Given: F 2 < PH 3 < H 2 O
   Restatement: Discuss boiling point (BP) order.
   General Trends
   
   
   • BP is a result of the strength of intermolecular forces—the forces betweenmolecules.
   
   
   • A direct relationship exists between the strength of intermolecular forces and the BP:
     The stronger the intermolecular force, the higher the BP.
   
   
   • Relative strength of intermolecular forces: H bonds > dipole forces > dispersion
     forces.
   
   
   
   

Answers and Explanations for the Practice Test