1. Accelerated Chemistry Molecular Composition of Gases
5/20/2019
Chapter 11
Molecular Composition of Gases
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

2. First, Let’s Review Chapter 9-11
Accelerated Chemistry
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First, Let’s Review Chapter 9-11
Stoichiometric Calculations
The coefficients in the balanced equation give the ratio of moles of reactants and products
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

3. Stoichiometric Calculations
Accelerated Chemistry
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Stoichiometric Calculations
From the mass of Substance A you can use the ratio of A and B to calculate the mass of Substance B formed (if it’s a product) or used (if it’s a reactant)
Mole
Grams
Liters
Molecules
Coefficients
Molecules
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

4. Stoichiometric Calculations
Accelerated Chemistry
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Stoichiometric Calculations
C6H12O6 + 6 O2  6 CO2 + 6 H2O
How many grams of H2O can be made from 1.00g C6H12O6?
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

5. 11.1 Volume - Mass Relationships of Gases
Accelerated Chemistry
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11.1 Volume - Mass Relationships of Gases
Last chapter, we related the volume and the mass of a gas:
Reaction Stoichiometry
Note: a “stoich” problem can be recognized by the fact that information from one substances is given and the problem is asking about a different substance.
The Combined Gas Law
Let’s compare the volumes of gases in two example problems…
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

6. …Reaction Stoichiometry at standard conditions
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…Reaction Stoichiometry at standard conditions
Example 1 - How many L of Oxygen are needed to react with 50.0L of Hydrogen at STP?
2H2(g) + O2(g) ---> 2 H2O(g) x
1 mole H2
22.4 L H2
1 mole O2
2 moles H2
22.4 L O2
50.0 L H2 =
25.0 L O2
We are comparing the mass and volume of a gas at standard conditions (STP) using “stoich” 1 3 2
But...we can skip steps 1 and 3.
Why? All gases take up the same amount of space at STP.
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

7. …The Combined Gas Law at nonstandard conditions
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…The Combined Gas Law at nonstandard conditions
Example 2 - Calculate the volume of one mole of H2 at 20ºC and 1000 torr.
P1 = 760 torr P2 = 1000 torr
T1 = 273K T2 = 293K
V1 = 22.4 L V2 = ?
V2 = 18.3 L H2
We can compare the volume and mass of a gas at non-standard conditions using the Combined Gas Laaw
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

8. Accelerated Chemistry
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The Ideal Gas Law
We solved example 2 using the combined gas law – we could have also used the ideal gas law
A relationship between pressure, volume, temperature and the # of moles of a gas
PV = nRT
n = # of moles
in Liters
Volume
R = ideal gas constant
Kelvin
Temp.
1 atm
1 atm = 760 torr
Pressure
at STP,
R = (1atm)(22.4L / 1mol)(273K)
= L · atm / mol· K
Crash Course Ideal Gas Law
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

9. Deriving the Ideal Gas Law
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Deriving the Ideal Gas Law
Let’s derive the ideal gas law and gas constant...
Volume is proportional to 1/P (as P is reduced, the V increases)
V is proportional to T (as T increases, the V increases)
V is proportional to n (as more moles are added, the V increases)
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

10. Accelerated Chemistry
5/20/2019
In general…
The combined gas law, P1V1 = P2V2
T T2
is used for changing conditions.
The Ideal Gas Law, can be introduced when you have problems containing:
one set of conditions
solving for grams
solving for moles
calculating molecular weight (molar mass)
calculating density
involving stoichiometry and non-STP conditions
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

11. Accelerated Chemistry
5/20/2019
One Set of Conditions
Example: Calculate the volume of 1.00 mole of Hydrogen at 20.0 ˚C and torr
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

12. Accelerated Chemistry
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Solving for Moles
Example: A sample of CO2 in a 10.0 L container at 293K exerts a pressure of 50,000. torr. How many moles of CO2 are in your sample?
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

13. Accelerated Chemistry
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Solving for Grams
Example: Calculate the number of grams of helium in a 6.0 liter cylinder at 27˚C and 800. torr.
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

14. Accelerated Chemistry
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Calculating M.W.
Example: If 18.0 grams of a gas at 380 torr and K occupies 44.8 L, what is the molecular weight of the gas?
Step 1: Identify the m.w. formula.
Step 2: Now, use PV = nRT to solve for n (the number of moles).
Step 3: Plug this value into the molecular weight (m.w.) equation.
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

15. Calculating Density at STP
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Calculating Density at STP
Example: Find the density of carbon dioxide at STP.
Step 1: Identify the density formula.
Step 2: If it is assumed that one mole of CO2 is present, then the mass can be calculated from the periodic table and the volume at STP is 22.4 L.
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

16. Calculating Density (not at STP)
Accelerated Chemistry
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Calculating Density (not at STP)
Example: Find the density of carbon dioxide at K and 4.00 atm.
Step 1: Identify the density formula.
D = m/V
Step 2: Assume one mole of CO2. Thus, CO2 weighs 44.0 grams. Plug this into the Density formula.
Step 3: Because the conditions are non-STP values, 22.4 L can’t be used. So, use PV=nRT and solve for V (assume one mole).
Step 4: Plug this value into the density equation.
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

17. Deviations from Ideal Behavior
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Deviations from Ideal Behavior
Real gases do not behave according to the KMT - Why?
Real gases have molecules that occupy space
Real gases have attractive and repulsive forces
Ideal gases conform exactly to the KMT
no such gas exists
gases only behave close to ideally at low P and high T.
At low T and high P, gases deviate greatly from ideal behavior.
Some gases are close to ideal (if they are small and nonpolar):
H2, He, Ne (these are small and nonpolar!!!)
O2 and N2 are not too bad
NH3, H2O are not even close to ideal
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

18. Stoichiometry of Gases
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Stoichiometry of Gases
At STP, there is nothing new here.
The only thing new is that 1 mole = 22.4 L must be adjusted if not at STP.
Problems for Stoichiometry of Gases include converting:
Grams to Liters
Liters to Grams
Liter to Liter
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

19. Accelerated Chemistry
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Grams to Liters
Example: How many liters of O2 are generated when 50.0 grams of sodium chlorate decomposes at atm and 20.0˚C?
2NaClO3(s) heat > 2NaCl(s) O2(g)
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

20. Accelerated Chemistry
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Liter to Grams
Example: If a lawn mower engine generates L CO2 on a lovely Sunday afternoon (.996 atm and C0 )- how many grams of octane were consumed?
2C8H18(l) O2(g) > CO2(g) H2O(g)
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes

21. Liter to Liter (not at STP)
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Liter to Liter (not at STP)
Example: How many L of carbon dioxide can be made from the combustion of 2.00 L of propane (C3H8) at 500. K and 3.00 atm?
C3H8(l) O2(g) > 3CO2(g) H2O(g)
Chapter 12 - Ideal Gas Law NotesChapter 12 - Ideal Gas Law Notes