April 30 Discussion
Q1, P1: Ideal Gas Laws
Q2, P2: Heat
Final Review Questions
(posted on Oncourse),
particularly problems 18-27
Q1: Ideal Gas Law
Which of the following is a consequence of the
ideal gas law, PV=NkT=nRT?
A)
B)
C)
D)
Boyle’s law, PV=constant
Charles’ law, V ~ T for constant P
Gay-Lussac’s law, P ~ T for constant V
All of the above.
P1: Air in a Scuba Tank
A scuba tank, when fully charged, has a pressure of
195 atm at 20°C. The volume of the tank is 11.3 L.
(a) What would the volume of the air be at 1.00 atm
and at the same temperature? (b) Before entering
the water, a person consumes 2.0 L of air in each
breath, and breathes 12 times a minute. At this rate,
how long would the tank last?
Q2: Transfer of Heat
Two 1kg blocks of different materials are put in thermal
contact with one another, but are otherwise isolated.
Block 1 is initially at temperature 100oC, and block 2 is
at 130oC. The specific heat of block 1 is twice that of
block 2. After the two blocks equilibrate, the final
temperature is:
A) 105oC
B) 110oC
C) 115oC
D)120oC
E) 125oC
The heat into the colder object equals the heat out of
the warmer one. Use Q=mcΔT.

T
c
1
2 1
mc

T


mc

T



1
1
2
2

T
2
2 c
1
1



T

100


T

130
F
F
2
So TF=110oC.
P2: Preparing for Surgery
You are helping a friend who is a veterinarian to do some minor
surgery on a cow. She has asked you to sterilize a scalpel and a
hemostat by boiling them for 30 minutes. You boil them as ordered
and then quickly transfer the instruments to a well insulated tray
containing 200 grams of sterilized water at room temperature (23 oC)
which is just enough to cover the instruments. After a few minutes the
instruments and water will come to the same temperature, but will
they be safe to hand to your friend without being burned? You are
both wearing surgical rubber gloves, but they are very thin. You know
that both the 50 gram scalpel and the 70 gram hemostat are made
from steel which has a specific heat of 450 J / (kg oC). Take the specific
heat of water to be 4200 J / (kg oC).
We want to know what is the final temperature of the instruments.
Since they were immersed in boiling water, their initial temperature
was 100oC. Heat out of the instruments = heat into water.
minstruments = (50+70)g = .12kg
csteel = 450J/kg-oC
mwater = .2kg
cwater = 4200/kg-oC
minstruments csteel (TF – 23)= mwater (100-TF)
Solve this to find TF = 27.7oC or 82oF
…perfectly safe to touch. Water has a very
large specific heat, so is good at cooling.