Name_____________________
Section: _____
1. Which takes more energy, melting a solid (at it’s melting point) or boiling the same mass of
the liquid (at it’s boiling point)? Explain your answer.
Boiling the liquid takes much more energy, because the IMF’s holding the molecules
together must be completely overcome for the liquid to turn to vapor. Melting a solid only
requires enough energy per mole so that the particles can move relative to eachother.
2. While cooking a pot of noodles, your friend asks if you’ve added salt to the water to make it
boil faster or to make the pasta cook faster. What is your answer?
Your answer is that you like to waste salt. As calculated in class, the amount of salt to raise
the bp of water even 1 K is way more than anyone would use, and the increase is negligible.
3. What type of intermolecular forces exist between molecules of:
a. hexane (CH3CH2CH2CH2CH2CH3) London only
O
║
b. acetone (CH3CCH3) London, dipole-dipole
4. Which solution will have a lower freezing point, 10 g of sodium acetate in 100 mL water or
10 g of sucrose in 100 mL water?
The solution of sodium acetate will have a lower freezing point. There are more moles, and
more particles (2 ions per mole).
5. Which boiling point do you expect to be higher, that of CH3CH2OCH2CH3 or
HOCH2CH2OH? Briefly describe your reason.
Ethylene glycol is higher, 200 C vs. 37 C. The hydrogen bonding IMF’s keep ethylene
glycol molecules as liquid more effectively than the dipole-dipole forces in the ether.
6. The Kf of water is 1.86 C/m. What is the freezing point of a solution made by adding 75.0 g
of CaCl2 to 1.00 L of water?
The solution is 2.04 m in ions, and Tf = –3.8 C.
7. What are the values fo molarity, molality and mole fraction of an aqueous solution that is
15.0% NaCl by mass?
Mole fraction = 0.0516 NaCl
m = 6.04 for ions, 3.02 for NaCl
M = about 2.8 M, assuming density is 1.1 g/mL