BELLARMINE COLLEGE
DEPARTMENT OF CHEMISTRY AND PHYSICS
CHEM115 - GENERAL, ORGANIC, AND BIOLOGICAL CHEMISTRY II
EXAM #1
ANSWER KEY
2/10/00
Do all the following questions. Please read each question carefully and apportion your time according to the relative
worth of the question (shown in parenthesis after the question). Write legibly. Marks will be deducted for unclear,
messy and illegible work. You must show all calculations (including units) where appropriate. Also, remember to
show answers to the correct number of significant figures. Answer in the space provided after each question. The
total marks for this exam is 50.
1.
You are at the beach on a hot summer's day. Why is it that the beach sand gets so hot that it will burn
unprotected feet, but the water remains cool? (5)
The specific heat of sand is much less than the specific heat of water (water has one of the
highest specific heats known). Therefore, it takes much less energy to heat up sand than it
does water. Also, a factor is that there is an awful lot more water in the ocean than there is
sand on the beach.
2.
You have just eaten a quarter-pounder cheeseburger, French fries, and a chocolate shake. The following table
shows the nutrition content of these food items:
ITEM
Cheeseburger
French fries
Chocolate shake
PROTEIN (g)
31
3
11
FAT (g)
29
11
9
CARBOHYDRATE (g)
34
26
60
The caloric value for fats is 9 kcal/g, and 4 kcal/g for carbohydrates and proteins. If a 70.0-kg person expends
60 kcal/hr sleeping and 200 kcal/hr walking, how long would it take you (assuming for the sake of this question
only that you weigh 70.0 kg) to "burn off" the kcal in this meal by a) sleeping and b) walking? (9)
Total protein = 45 g, total fat = 29 g, and total carbohydrate = 120 g.
Calories from protein = 45 g x 4 kcal/g = 180 kcal
Calories from fat = 49 g x 9 kcal/g = 441 kcal
Calories from carbohydrate = 120 g x 4 kcal/g = 480
Total calories = 1101 kcal
SLEEPING: 1101 kcal x hr/60 kcal = 18 hrs
WALKING: 1101 kcal x hr/200 kcal = 5.5 hrs
3. When 1.0 g of gasoline burns, 1.15 x 10 4 cal of energy are released. If the density of gasoline is 0.743 g/mL,
how much energy (in kcal) could be obtained from 11.9 gallons of gasoline? (1.0 gallon = 3.784 L)
(5)
11.9 gal x (3.784 L/1.0 gal) x (1000 mL/L) x (0.743 g/mL) x (1.15 x 104 cal/g) x (kcal/1000cal)
= 3.8 x 105 kcal
4.
a.
b.
Benzene is an organic liquid once commonly used as a solvent. However, it has since been proved to be a
carcinogen and, therefore, its use has been severely curtailed. The melting point of benzene is 5.5 oC its boiling
point is 80.1oC.
Sketch a heating curve for benzene from 0oC to 100oC. (3)
How much energy (in kcal) would be needed to melt a 15.0-g block of frozen benzene at 5.5oC, heat the
resultant liquid benzene to its boiling point, and then convert the liquid to benzene vapor at 80.1 oC? The heat of
vaporization of benzene is 94.1 cal/g, the heat of fusion of benzene is 30.45 cal/g and the specific heat of
benzene is 0.416 cal/g.oC. (8)
MELTING: heat = 15.0 g x 30.45 cal/g = 456.75 cal
HEATING: heat = 15.0 g x 74.6oC x 0.416 cal/g.oC = 465.504 cal
BOILING: heat = 15.0 g x 94.1 cal/g = 1411.5 cal
TOTAL: 2333.754 cal = 2.33 kcal (to 3 SF)
5.
Gasoline reacts with oxygen in the air exothermically, yet there is no reaction when you are filling up your car
with gasoline at the filling station where there is plenty of air for the gasoline to come into contact with. Why
is there no reaction? Use an energy diagram to illustrate your answer. (5)
At room temperature, oxygen and gasoline molecules don't have enough energy when
they collide to get over the activation barrier. It is only when a spark is supplied (as
with a spark plug in a car engine) that enough energy is supplied to get the reaction
going. Since the reaction is exothermic, some of the energy released when the first
molecules react goes to helping the rest of the unreacted gasoline and oxygen molecules
get over the activation barrier, i.e., a chain reaction ensues.
6.
A syringe has a volume of 10.0 cc (10.0 mL) at a pressure of 760.0 mm Hg. If the tip of the syringe were
blocked so that air couldn't escape, what pressure would have to be exerted to decrease the volume to 2.00 cc?
(5)
P1V1 = P2V2
P1 = 760.0 mm Hg, V1 = 10.0 cc, P2 = ?, and V2 = 2.00 cc
P2 = (P1V1)/V2 = (760.0 mm Hg x 10.0 cc)/2.00 cc = 3800 mm Hg (3.80 x 103 mm Hg to 3 SF)
7.
A balloon filled with helium has a volume of 1.50 L at sea level where the pressure is 760.2 mm Hg. Keeping
the balloon in an insulated container so that its temperature remains constant, you climb to the top of Mt.
Whitney in California where the atmospheric pressure is 439.6 mm Hg. What will be the volume of the
balloon? (5)
P1V1 = P2V2
P1 = 760.2 mm Hg, V1 = 1.5 L, P2 = 439.6 mm Hg, and V2 = ?
V2 = P1V1/P2 = (760.2 mm Hg x 1.5 L)/439.6 mm Hg = 2.59 L (to 3 SF)
8.
A hot-air balloon has a volume of 875.0 L. When the balloon is heated to 56.0 oC, its volume changes to 955.0
L. What was the temperature of the balloon before it was heated? (5)
T1/V1 = T2/V2
V1 = 875.0 L, T1 = ?, V2 = 955.0 L, T2 = 56 (+ 273) = 329 K
T1 = T2V1/V2 = (329 K X 875.0 L)/955.0 L = 301 K (-273) = 28.4oC (to 3 SF)