5 Steps to a 5 AP Chemistry

1. C—This question relates to the combined gas
   law: P 1 V 1 /T 1 = P 2 V 2 /T 2. Since the pressure
   remains constant, the pressures may be removed
   from the combined gas law to produce Charles’s
   law: V 1 /T 1 = V 2 /T 2. This equation may be
   rearranged to: T 2 =V 2 T 1 /V 1. The doubling of
   the volume means V 2 = 2 V 1. On substituting:
   T 2 = 2 V 1 T 1 /V 1 ; giving T 2 = 2 T 1. The identity of
   the gas is irrelevant in this problem.


2. E—This problem depends on the ideal gas equa-
   tion: PV=nRT. R, V, and Tare known, and by
   using the partial pressure for a gas, the number of
   moles of that gas may be determined. To convert
   from moles to mass, the molar mass of the gas is
   needed.


3. B—Since Tand P are known, and since the
   moles (n) can be determined from the masses
   given, this question could use the ideal gas equa-
   tion. The number of moles of each gas is 0.50.
   Equal moles of gases, at the same Tand P, have
   equal volumes. Equal volume eliminates answer
   choice D. Equal volume also means that the
   greater mass has the greater density, eliminating
   choice A. Equal moles means that the numbers
   of molecules and atoms are equal, eliminating
   choice E. The average kinetic energy of a gas
   depends on the temperature. If the temperatures
   are the same, then the average kinetic energy is
   the same, eliminating C. Finally, at the same
   temperature, heavier gases travel slower than
   lighter gases. Nitrogen is lighter than argon, so it
   travels at a faster average speed, making B the
   correct answer. You may find this reasoning
   process beneficial on any question in which you
   do not immediately know the answer.
   4. A—This experiment requires the ideal gas equa-
   tion. The mass of the solid is needed (to convert
   to moles); this eliminates answer choice D. The
   volume, temperature, and pressure must also be
   measured during the experiment, eliminating
   choices B, C, and E. The measured pressure is
   the total pressure. Eventually the total pressure
   must be converted to the partial pressure of the
   gas using Dalton’s law. The total pressure is the
   sum of the pressure of the gas plus the vapor
   pressure of water. The vapor pressure of water
   can be looked up in a table when the calculations
   are performed (only the temperature is needed to
   find the vapor pressure in a table). Answer A is
   correct.
   5. C—Real gases are different from ideal gases
   because of two basic factors (see the van der
   Waals equation): molecules have a volume, and
   molecules attract each other. The molecules’
   volume is subtracted from the observed volume
   for a real gas (giving a smaller volume), and the
   pressure has a term added to compensate for the
   attraction of the molecules (correcting for
   a smaller pressure). Since these are the only two
   directly related factors, answers B, D, and E are
   eliminated. The question is asking about volume;
   thus, the answer is C. You should be careful of
   “NOT” questions such as this one.
   6. C—A balanced chemical equation is needed:

2 Al +6 HCl → 2 AlCl 3 +3 H 2

The reaction produced 33.6L/22.4 L or

1.50 mol at STP. To produce this quantity of

hydrogen, (2 mol Al/3 mol H 2 ) ×1.50 moles

H 2 = 1.00 mol of Al is needed. The atomic

weight of Al is 27.0; thus, 27.0 g of Al are

required.

7. D—The partial pressure of any gas is equal to its
   mole fraction times the total pressure. The mole
   fraction of carbon monoxide is [0.30/(0.60 +
   0.30 +0.10)] =0.30, and the partial pressure of
   CO is 0.30 ×0.80 atm =0.24 atm.

Gases  117

 Answers and Explanations