1. Homework 10 posted and due Friday 13 April

2. P1V1 =P2V2 T1 T2 T must be in Kelvin (K) T(K)= [T(oC)+273]
When n is constant…use the Combined Gas Law:
P1V1 =P2V2
T T2
T must be in Kelvin (K)
T(K)= [T(oC)+273]

3. COMBINED GAS LAW PROBLEMS…BOARD WORK (CONT.)
The volume of a piston at fixed pressure changes as it is
cooled from 500 oC to 250oC. If the final volume is 6.76 L,
what is the initial volume ?
T(K)= [T(oC)+273]
V1=10 L

4. An ideal gas in a fixed volume and an initial pressure of 10 atm and initial T of 177 C has a final pressure of 3.33 atm. What is the final T(K)? T(K)= [T(o C)+273]
150 K
59 K
450 K
531 K

5. 4. Autoclaves are essentially pressure cookers. At 1 atm,
COMBINED GAS LAW PROBLEMS…BOARD WORK (CONT.)
Autoclaves are essentially pressure cookers. At 1 atm,
steam has a temperature of 100o C. Would you expect the
pressure to double if the autoclave to attains a steam
temperature of 200oC ?
a) NO…must convert C K…ratio is not 200/100
What pressure do you actually expect to reach at 200 oC?
Recall: [T(K) =273 +T(oC)]
=1.73 atm
373.15

6. Gas at constant volume and moles is initially at 27 oC and a pressure of 1 atm. What temperature (in C) must we reach to attain a pressure of 2 atm ? [T(K) =273 +T(oC)]
54 oC
127 oC
300 oC
427 oC

7. Not all combined gas law problems are easy…

8. Final P = ??? 0.483 atm Trickier problem 2.00 L H2 at 0.625 atm
Stopcock closed
2.00 L H2 at atm
1.00 L N2 at atm
Final P = ???
Stopcock open
0.483 atm

9. Need more practice with Combined Gas Law ????

10. When n varies…. Blowing up a balloon What varies ? => V up
n up ?
=> V up
n, V (1st time n changes)
n down ?
=> V down
=> P and T are constant
Avogadro’s Law (pp )
V=an V n

11. What happens if we let n vary too ??

12. Heating/cooling coils
Ideal Gas Law: letting T,P,V, n and gas ID all vary
(pp )
P (piston head)
Hypothetical Gas Property testing apparatus
piston walls
insulation T n
V (varies
GAS
GAS VALVE
Heating/cooling coils

13. Gas ID (and size) not important (!!)
Ideal Gas derived: Why gas identity not important
Vary gas and fix three out of four gas variables…
..see what happens
Variables fixed at constant values (`STP’)
Gas
varied
T(OC) P(atm) n(moles)
V(obs)
H2(2)
He(4)
N2(28)
CO2(44)
SF6 (146) 0o 0o 0o 0o 0o
STP =Standard Temperature & Pressure ( and n=1)
Gas ID (and size) not important (!!)

14. Ideal Gas Law derived (cont.): origin of R
What happens if we hold different sets of three variables constant and watch the fourth for a given gas ?
Anything constant ??
PV/nT
P(atm) V(L) T(K) n(moles)
16.43 1
24.65
0.406
101 3

15. PV = nT
=R (atm L)
(mol K )
Or…
PV =nRT

16. The ideal gas law leads to :
molecular masses
verification of stoichiometries

17. 1. An 11 gram sample of a gas occupies 2. 0 liters at 2
1. An 11 gram sample of a gas occupies 2.0 liters at atm and 200 K. What is the molecular mass of the gas ? (R= )
 44 g/mol

18. 9 grams of an unknown material occupies 2 L at 1
0.9 grams of an unknown material occupies 2 L at 1.23 atm at a temperature of 600 K. What is the molecular mass of the unknown? (R=0.082)
18 g/mol
12.3 g/mol
2.46 g/mol
36 g/mol

19. 3. A 1.5 liter can of gas reaches a pressure of atm at 500 K. How many molecules of gas are in the can ? (R= atm L/K mol)
1 mol count =6.02*1023
1.0*1024

20. A 4 liter cylinder at 300 K reaches a pressure of 3. 075 atm
A 4 liter cylinder at 300 K reaches a pressure of atm. How many gas molecules are in the cylinder ? (R=0.082 atm L/K mol. 1 mol=6*1023 molecules)
0.042*1023
3*1023
3*1022
12*1023