5 . D
This question is an application of Avogadro’s principle, which states that at a constant
temperature and pressure, all gases will have the same number of moles in the same volume.
This is true regardless of the identity of the gas. Thus, if there is 0.25 mol of He gas under one
set of conditions, there will likewise be 0.25 mol of chlorine gas under the same set of
conditions. The number of chlorine gas molecules is therefore 0.25 times Avogadro’s number:Note that the actual numerical value of the volume does not even come into play.6 . D
The average speed of a gas is defined as the mathematical average of all the speeds of the gas
particles in a sample. To answer this question, you must understand the Maxwell-Boltzmann
distribution curve, which shows the distribution of speeds of all the gas particles in a sample at
a given temperature. The distribution curve is a bell-shaped curve that flattens and shifts to
the right as the temperature increases. The flattening of the curve means that gas particles
within the sample are traveling at a greater range of speeds. As a result, a smaller proportion
of the molecules will move at exactly the new average speed.
7 . A
A gas weighing 25 g/mol will have a density of (25 g/mol) / (22.4 L / mol) = (25/22.4) g/mol at
STP. The density at 76 torr and 37ºC is found by calculating the change in volume of a mole of
gas under these conditions:
Therefore the density of the gas is 25g / 254 L = 0.1 g/mol. Alternatively, we can use the
equation given earlier: 8 . 8.88
First, find out the volume of one mole of gas at the pressure and temperature given: