1. Plan for Wed, 8 Oct 08 Mistake in Exam 1 key Lecture
Graphical LR problem: #13 in V1, #5 in V2
Keys say the answer is B…the answer is really D
Bring your exam to me by Monday to get these points back
Lecture
Ideal gas law and gas density (5.3)
Gas stoichiometry (5.4)
Don’t forget about Friday’s quiz.

2. Standard T & P (STP) At STP, 1 mol of any ideal gas occupies 22.4 L
T = 273 K (0oC)
P = 1 atm
= kPa
= bar
(1 bar = 100 kPa)
22.4 L

3. PV = nRT Example
What is the pressure exerted by 1.00 x 1020 molecules of N2 gas in a 305 mL flask at 175oC?
P = ?
V = 305 mL = L
n = 1.00 x 1020 molcs N = mol N2
R = L.atm/mol.K
T = 175oC = 448 K
1 L
1000 mL
1 mol N2
NA molcs N2

4. What is the pressure exerted by 1
What is the pressure exerted by 1.00 x 1020 molecules of N2 gas in a 305 mL flask at 175oC?
P = ?
V = L
n = mol N2
R = L.atm/mol.K
T = 448 K

5. PV = nRT Example
A 50.0 L gas cylinder containing 21.5 atm of gas is connected to an odd shaped tank. After the gas in allowed to expand into the tank, the pressure on the cylinder reads 1.55 atom. What is the volume of the tank?
Petrucci, pg. 182

6. It will depend strongly on T and P.
Gas density
We can use PV = nRT to determine the density of gases.
What are the units of density?
mass/volume
What does this suggest about gas density?
It will depend strongly on T and P.
Contrast with liquids and solids, whose densities depend somewhat on T, but far less on P.

7. Gas Density Example
What is the density of O2 gas in g/L at 25oC and atm?
Calculate # moles in 1 L, use MM of O2 to get g, divide by V
P = atm
V = 1 L
n = ?
R = L.atm/mol.K
T = 25oC = 298 K

8. What is the density of O2 gas in g/L at 25oC and 0.850 atm?
Calculate # moles in 1 L, use MM of O2 to get g, divide by V
P = atm
V = 1 L
n = ?
R = L.atm/mol.K
T = 298 K
Note: A value for gas density is meaningful only if accompanied by the T and P at which it was measured.

9. Gas Density and Molar Mass
In our previous example, we calculated the density of O2(g).
How would our calculation change if we wanted the density of Cl2(g)? Or Br2(g)?
Did this calculation depend at all on the nature of O2(g)?
No! It only depends on the T, P, and V.
Remember Avogadro’s Law? Two equal volumes of an ideal gas at the same T and P will contain the same number of moles of gas, regardless of the identity of the gas.
So, a 1 L sample of either Cl2(g) or Br2(g) at atm and 298 K will contain mol of gas particles.

10. Gas Density and Molar Mass
Density of 1 L of Cl2(g) at atm and 298 K:
Density of 1 L of Br2(g) at atm and 298 K:
Therefore, the density of an ideal gas depends on T, P, V, and Molar Mass

11. Gas Density and Molar Mass
We can develop an expression relating density and MM using PV = nRT.
Substituting:
We can use the density of a gas to determine its molar mass.
Density!!

12. Gas Stoichiometry Many chemical reactions involve gases.
Gases are often measured in terms of volume (given T and P) or pressure (given V and T) rather than mass.
If we assume that the gasses behave ideally, we can use the Ideal Gas Law to determine the number of moles of gas consumed or produced.

13. Gas Stoichiometry Example
What volume of N2, measured at 735 mmHg and 26oC, is produced when 70.0 g NaN3 is decomposed?
2 NaN3(s)  2 Na(l) + 3 N2(g)
a) find moles of NaN3
b) use mole ratio to find moles of N2
c) use PV= nRT to find volume of N2