Chemestry exercise

Chemistry 2045-10F Fall 2022Exam 4
Name:__________________________________
Directions:
This is a 75-minute exam. You may a calculator and either pen or pencil. Supplementary
information is provided at the end of the exam. Best of luck. There is scratch paper provided – be
sure you have all 8 pages.
1. Rank the following from highest boiling point (1) to lowest boiling point (4). Explain
your choice [16 pts]
2. Given the phase diagram for a material with freezing point at 7.2 oC and boiling point at
59 oC – identify A though H (solid, liquid, gas, supercritical fluid, sublimation curve,
freezing curve, vaporization curve, triple point, & critical point). Please note: you are
able to use the same answer more than once and you may not use all of listed words.
[16 pts]
3. Although steel is denser than water, a steel needle or paper clip placed carefully
lengthwise on the surface of still water can be made to float. Explain at a molecular level
how this is possible. [8 pts]
4. FSW question:
Which of the following molecules would have the strongest intermolecular forces? [10 pts]
a)
b)
c)
d)
e)
CH4
SiH4
GeH4
NH3
PH3
5. AP Chemistry question:
The phase diagram for the pure substance X is shown above. The temperature of a sample
of pure solid X is slowly raised from 10ºC to 100ºC at a constant pressure of 0.5 atm.
What is the expected behavior of the substance?? [10 pts]
A. It first melts to a liquid and then boils at about 70ºC.
B. It first melts to a liquid and then boils at about 30ºC.
C. It melts to a liquid at a temperature of about 20ºC and remains a liquid until the
temperature is greater than 100ºC.
D. It sublimes to vapor at an equilibrium temperature of about 20ºC.
E. It remains a solid until the temperature is greater than 100ºC.
6. Homework question: [20 pts]
Explain the following observations:
(a) It takes longer to cook an egg in Ft. Davis, Texas (altitude, 5000 feet above sea level)
than it does in Boston (at sea level).
(b) Perspiring is a mechanism for cooling the body.
7. How many moles of gaseous water exist if 40.0 kJ of energy was added to a cup of hot
water (250 g, 70 oC)? Assume Cs of liquid and gaseous water are 4.186 kJ/kg•K and
1.996 kJ/kg•K respectively while the ΔHvap = 40.65 kJ/mol. [10 pts]
8. What is the final temperature of the cup of hot water described in 7 if it was instead
provided with a 50g-ice cube beginning at -5 oC? Assume Cs of ice is 2.09 kJ/kg⋅K and
the ΔHfus = 6.01 kJ/mol [10 pts]
PV = nRT
Thermochemistry:
𝑞 = 𝑚 × 𝐶𝑠 × 𝛥𝑇
𝑞𝑠𝑦𝑠 = −𝑞𝑠𝑢𝑟𝑟
𝑞 = 𝑛 × 𝛥𝐻
Gas constants (R):
8.314 J⋅K−1⋅mol−1
8.314 m3⋅Pa⋅K−1⋅mol−1
1.987×10−3 kcal⋅K−1⋅mol−1
62.36 L⋅Torr⋅K−1⋅mol−1
8.206×10−5 m3⋅atm⋅K−1⋅mol−1
0.08206 L⋅atm⋅K−1⋅mol−1