Bell Ringer

1
Bell Ringer
298 K
A sample of nitrogen occupies 10.0 liters at 25ºC
and 98.7 kPa. What would be the volume at 20ºC
and 102.7 kPa? A 7.87 L B 9.45 L C 10.2 L D
10.6 L
A sample of nitrogen occupies 10.0 liters at 25ºC
and 98.7 kPa. What would be the volume at 20ºC
and 102.7 kPa? A 7.87 L B 9.45 L C 10.2 L D
10.6 L
293 K
T2
T2
P2
P2
P1 V1 T2
V2

T1 P2
(98.7 kPa) (10.0 L) (293 K)
V2

(298 K) (102.7 kPa)
2
Gas Laws Quiz
Good Luck!!
3
For Next Class

• Homework
• Gas Laws Packet 2, problems 1-10
• Quiz next class on Ideal Gas Law, Partial
  Pressures, and Density
• 5 questions 22 points total
• 2 short answer/FITB (2 points each)
• 3 math problems (6 points each)

4
The Ideal Gas Law Co.

• Ms. Besal
• 3/14/2006

5
A Reminder
assume
ideal

• We that we live in an
  world where
• Gas particles have no mass
• Gas particles have no volume
• Gas particles have elastic collisions

These assumptions are used when trying to
calculate the AMOUNT of a gas we have!
6
Why are these assumptions important?
PV nRT
Image source thefreedictionary.com
7
PV nRT
The Ideal Gas Law
RESSURE
OLUME
MOLES OF GAS
GAS CONSTANT
EMPERATURE
Image source popartuk.com
8
The MysteRious R

• R is a constant (doesnt change).
• Number value of R depends on other units.
• Units of R are a combination of many units.

Image source toysrusemail.com
9
Ummm What?
PV nRT
Solve for R
P V
R
n T
Plug in units
(mm Hg) (L)
(kPa)
(atm)
R
(mol) (K)
10
Gas Laws, Gas Laws Everywhere!
Charles' Law
Boyle's Law
P1 x V1 P2 x V2
Combined Gas Law
Ideal Gas Law
Used with CHANGING CONDITIONS
P V n R T
Used with only ONE SET OF CONDITIONS
11
When to Use PV nRT

• Calculating amount of gas in moles
• Calculating P, V, or T if moles of gas are known.
• IMPORTANT! We must have 3 out of 4 pieces of
  information
• P
• V
• n
• T

12
Practice with the Ideal Gas Law

1. A gas sample occupies 2.62 L at 285ºC and 3.42
   atm. How many moles are present in this sample?

PV nRT
P V
n

R T
(3.42 atm)
(2.62 L)
n


0.196 mol
(558 K)
13
But Lets Be Practical
We dont usually measure in moles! We usually
measure quantities in GRAMS!
PV nRT
PVM gRT
14
PVM gRT
RESSURE
OLUME
OLAR MASS OF GAS (g/mol)
RAMS OF GAS
GAS CONSTANT
EMPERATURE
Image source popartuk.com
15
Practice with the Ideal Gas Law
A balloon is filled with 0.2494 g of helium to a
pressure of 1.26 atm. If the desired volume of
the balloon is 1.250 L, what must the temperature
be in ºC?
P V M
PVM gRT
T

g R
(1.26 atm)
(1.250 L)
T

308 K

- 273
(0.2494 g)
35 ºC
16
PVnRT vs. PVMgRT

• Use PVnRT when
• You are given moles in the problem.
• You are searching for moles as an answer.
• Use PVMgRT when
• You are given grams in the problem.
• You are searching for grams as an answer.

17
What Else Happens Under Unchanging Conditions?
At constant V and T, pressure is easy to
calculate!
The sum of the individual pressures is equal to
the total pressure.
Total Pressure Pressure of gas 1 Pressure of
gas 2 Pressure of gas 3 Pressure of
gas 4
Ptotal P1 P2 P3
Dalton's Law of Partial Pressures
18
Partial Pressures Practice
A sample of hydrogen gas is collected over water
at 25ºC. The vapor pressure of water at 25ºC is
23.8 mmHg. If the total pressure is 523.8
mmHg, what is the partial pressure of the
hydrogen?
Ptotal PH2 PH2O

23.8 mm Hg
523.8 mm Hg
PH2


PH2
500.0 mm Hg
Source 2003 EOC Chemistry Exam
19
What do Changing Conditions Affect?
We have learned that we can change 3 variables
Temperature, Volume, and Pressure.
If MASS remains constant
But VOLUME changes
Then DENSITY CHANGES!
20
Two Types of Density Problems
At STP
Not at STP

• molar volume of any gas at STP

• Determine new volume (V2 ) using Combined Gas Law

22.4 Liters
STP Values
non-STP

• Density at STP

• Density at non-STP

molar mass
molar mass
molar volume
22.4 Liters
V2
21
Practice with Density Problems
Determine the density of ethane (C2H6) at STP
Determine the density of C2H6 at 3.0 atm and 41ºC.
V2 8.6 L
molar mass
D (at STP)
molar volume
V2 P1 V1 T2
molar mass 30.08 g
T1 P2
molar volume 22.4 L
V2 (1.0 atm) (22.4 L) (314 K)
3.5 g/L

(273 K) (3.0 atm)
30.08 g
D

1.34 g/L
22.4 L
V2 8.6 L
22
For Next Class

• Homework
• Gas Laws Packet 2, problems 1-10
• Quiz next class on Ideal Gas Law, Partial
  Pressures, and Density
• 5 questions 22 points total
• 2 short answer/FITB (2 points each)
• 3 math problems (6 points each)

23
What do Changing Conditions Affect?

• Density

• Stoichiometry problems

So Far
Now
STP
Non-STP
24
Mass-Volume at Non-STP
Two parts to solving these problems

• Use Stoichiometry
• Use Gas Law