Catalyst
1. 10 L of an unknown gas has a mass of 10.8 grams at a temperature
of 310 K and 1.2 atm. What is the molar mass of this mass? What is
the identity of the gas?
Loose Ends
 Labs and Lab Write – Ups
 The only way you learn from labs is through the analysis
 These are not meant to torture. They honestly help with the content.
 E.g. Net Ionic Equation Lab
 Lab Reports
 Human Error – Don’t want to see it ever again in a report…you can’t
BS a teacher that made it through education using the same BS.
 Plagiarism
 New Lab Report/Write-Up Policy
 You call mom/dad/guardian to explain why your don’t value your
education
 No lab write up = No lunch/nutrition
 You will do the write up during lunch/nutrition, but you will not get
the points
J – TPS
 If the atmosphere at 120,000 ft contained oxygen, nitrogen, and
hydrogen at pressures of 0.0002 atm, 0.0003 atm, and 0.0001 atm,
then what is the pressure of the atmosphere at this height?
Lecture 4.2 – Partial Pressures and
Deviations from Ideality
Today’s Learning Targets
 LT 4.4 – I can discuss Dalton’s Law of Partial Pressures.
Additionally, I can calculate partial pressures for a gas mixture
 LT 4.7 – I can compare and contrast ideal and real gases. I can
discuss how the Van der Walls equation corrects for these
deviations from ideality.
Dalton’s Law of Partial Pressures
 The total pressure of a mixture of gases equals the sum of all the
partial pressures
 This means that we can add up all the individual pressures (partial
pressures) to get the overall pressure of a system.
Class Example
 A mixture of 6.00 g O2 (g) and 9.00 g CH4 (g) is placed in a 15.0 L
vessel at 0 oC. What is the partial pressure of each gas, and what is
the total pressure in the vessel?
Table Talk
 What is the total pressure exerted by a mixture of 2.00 g of H2 and
8.00 g of N2 at 273 K in a 10.0 L vessel.
Mole Fractions
 Because each gas acts independently even when mixed, we can
relate the amount of moles of gas in a mixture.
 The mole fraction (X) expresses the ratio of moles of gas to total
moles of gas in the system
moles of compound 1
n1
X1 =

total moles
nt
Class Example
 A study of the effects of certain gases on plant growth requires a
synthetic atmosphere composed of 1.5 mol percent CO2, 18.0 mol
percent O2, and 80.5 mol percent Ar. Calculate the partial
pressure of O2in the mixture if the total pressure of the
atmosphere is to be 745 torr.
Table Talk
 The composition of the atmosphere of Saturn’s moon Titan has
been estimated. The pressure on the surface of Titan is 1220 torr.
The atmosphere consists of 82 mol percent N2, 12 mol percent Ar,
and and 6.0 mol percent CH4. Calculate the partial pressure of
each gas.
Collecting Gas Over Water
 In most labs, we bubble gas through water and catch it in some
vessel.
 This is known as collecting a gas over water
 We must correct for the pressure of the water vapor that is present
in the air above where the gas was collected
Ptotal  Pgas  PH 2O
Pgas  Ptotal  PH 2O
Class Example
 When a sample of KClO3 is partially decomposed, O2 is produced
and the gas is collected over water. The volume of the gas collected
is 0.250 L at 26 oC and 765 torr. How many moles of O2 are
collected? NOTE – At 26 oC, the PH2O is 25 torr.
Table Talk
 Ammonium nitrate decomposes on heating to form N2 gas:
 NH4NO2 (s)  N2 (g) + 2H2O (l)
When a sample of NH4NO2 is decomposed
Collaborative Poster
 In a group of three, analyze and solve the problems given to you.
 Assign person A, B, and C in your group
 Person A – Identify the values for all variables and givens in the
problem
 Person B – Identify and write the equations that you will need to use
to solve these problems
 Person C – Solve the problem.
+
 PbI2
2. Cs2O + H2O  2CsOH
+
2+
3. 2 H + ZnS  H2S + Zn
1. 2
I
2+
Pb
Lab 8 – Molar Mass of Butane Gas
Lab Work Time
Work Time
 For the remainder of class, please begin your homework that will
be due Thursday/Friday
Closing Time
 Read 10.6 and answer essential questions on reading guide
 Complete book questions: 10.63, 10.65, 10.66, 10.69, 10.71,
10.73, and 10.74