Warm-Up 12/14/2016
•How many moles of nitrogen are in 135 L of
nitrogen gas at Standard Temperature and
Pressure(STP)?
•A) 4.82 moles of N2
•B) 5.53 moles of N2
•C) 6.02 moles of N2
•D) 9.64 moles of N2
Dalton’s Law of Partial Pressure Continued
•So what if we said:
•ntotal = n1 + n2 + n3 + ….
•If each of the gases obeys the ideal gas equation,
we can write :
•P1 = n1 (RT/V) , P2 =n2(RT/V), P3= n3 (RT/V)
•Ptotal = ( n1 + n2 + n3 + ….) (RT/V) = ntotal(RT/V)
Practice
•A gaseous mixture made from 6.00 g O2 and
9.00 g CH4 is placed in a 15.0 L vessel at 0˚ C.
What is the partial pressure of each gas, and
what is the total pressure in the vessel
Practice
•What is the total pressure exerted by a
mixture of 2.00 grams of H2 and 8.00 grams
of N2 at 273 K in a 10.0 L vessel?
Collection of Gas by
Water Displacement
•Gases are often collected by bubbling them
through water
•This is known as “collecting a gas by water
displacement”
•The total pressure is equal to the gas’s pressure
plus the pressure of the water vapor
•Ptotal = Pgas + Pwater vapor(H2O)
Example Problem
•A quantity of gas is collected over water at
3
8°C in a 353 cm vessel. The manometer
indicates a pressure of 84.5 kPa. What is the
pressure of the gas if the vapor pressure of
water at 8°C is 1.1 kPa?
Example # 2
3
cm
A 450
sample of hydrogen is collect over
o
water at 12 C. The pressure of the hydrogen
and water vapor mixture is 78.5 kPa. What is
the partial pressure of the dry hydrogen gas?
Water vapor pressure at 12 ° C is 10.52 kPa
Graham’s Law
•Diffusion is the random scattering of gas
molecules
•It is the mixing of gas molecules
•Diffusion occurs when molecules migrate
from areas of high concentration to areas of
low concentration
•Example: If a bottle of ammonia is open, you
will soon smell the ammonia due to the
diffusion of gaseous ammonia particles
Graham’s Law Continued
•Graham determined a law that allows for the
rate of effusion to be measured
•Effusion occurs when gas molecules escape
from a small hole in the gas’s container
•Graham found that the rate of effusion of a
gas is inversely proportional to the square root
of its molar mass
Diffusion and Effusion
Graham’s Law Continued
• The larger a particle’s mass, the slower the particle will
effuse
• Graham’s Law is:
Rate A molar mass gas B
Rate B
=
molar mass gas A
Example #1
•What is the relative rate of diffusion of
Helium to Neon?
•Ans) 2.24
•Interpretation: Helium particles will diffuse
2.24 times faster than Neon particles
Additional Examples
•3. Compute the relative rate of diffusion of
argon to radon
•4. Compute the relative rate of diffusion of
helium to argon
•Ans 3) 2.36
•Ans 4) 3.16
Practice
A sealed can with an internal pressure of 721
Torr at 25 ˚C is thrown into an incinerator
operating at 755˚C. What will be the pressure
inside the heated can, assuming that container
remains intact during incineration?
Practice
How many molecules are in a gas that has a
pressure of 6 kPa, a volume of 5 L, and a
temperature of 127°C?
Practice
•What is the volume of 1.00 mole of a gas at
stadard temperature and pressure?
Practice
•A 113 L sample of helium at 27˚C is cooled
at constant to -78.0˚C. Calculate the new
volume of the helium.
Practice
•What volume of He is occupied by 2.35 mol
of He at 25˚C and a pressure of 0.980 atm?