Practice for Unit IV Test / Answer Key
AP Chemistry
Name_____________________________ Date_________________________
Multiple Choice
1) c
2) e
3) b
4) a
5) c The shape of the curve shows that the substance was super cooled (slowly cooled to a
temperature below its freezing point). The substance begins to become a solid at point
Q.
6) b Both density and molarity will change due to the slight expansion of water as it is
heated from 20 C to 90 C. Mole fraction will be unchanged.
7) a
8) d use the equation XA = PA/Ptotal
9) c
10) b As temperature of a gas increase, solubility of a gas decreases. As pressure above a
solution increases, temperature of a gas increases. Think of what happens to the
dissolved carbon dioxide in a can of cold soda, when you open the can and allow it to
warm up to room temperature.
11) d
12) d
13) a
14) a
15) a
16) e
17) a
18) b
19) a
20) c
21) b
22) d
Short Answer
1) Ne<CO<CH3NH2<NaF. Neon atoms are attracted to each other through London
dispersion forces. The low number of electrons in the atom causes these forces to be
weak. Carbon monoxide molecules are polar, in order to boil enough energy must be
used to overcome both London dispersion forces and dipole forces. Methyl amine has
the strongest IMF’s due to hydrogen bonding between molecules. Therefore even more
energy is required to break up the IMF/s and boil the substance. Finally, in order to boil
sodium fluoride, an ionic compound, enough energy must be provide to break the all of
the ionic bonds holding the ions together.
2) 18.4 M H2SO4, density = 1.84 g/ml; 5.20 M H2SO4, density = 1.38 g/ml
a. M1V1 = M2V2; (18.4)(V1) = (5.20)(1); V1 = 283 mL
b.
18.4g 1000ml
x
= 1840g solution;
1ml
1L
98g
18.4molH 2 SO4 x
= 1803gH 2 SO4 ;
1mol
1Lsolution x
mass percent = (mass H2SO4/total mass) x 100; (1803/1840)x100 = 98%
3) letter
a. Salt is spread on sidewalks to reduce the freezing point of water. Freezing point
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depression, a colligative property, depends on the number of solute particles
dissolved.
When added to water NaCl dissociates into two particles; CaCl2,
dissociates into three particles. Given equal moles of NaCl and CaCl2, CaCl2 will
have a greater effect on the freezing point and boiling point of water because
more ions are dissolved in the solution.
b. Boiling occurs when the vapor pressure is equal to the atmospheric pressure.
Because the vacuum pump in causing the atmospheric pressure to decrease,
atmospheric pressure will equal vapor pressure at a lower temperature. So the
water will boil at a lower temperature.
c. Iodine a non polar compound dissolves in non polar TTE. The result is a purple
solution with a density greater than the density of water. Because water is polar
it does not contain any dissolved iodine and is not miscible with TTE. Because of
the density differences water which is less dense is above the TTE/iodine
solution.
d. Perspiration involves the evaporation of water from the body.
H20(l)+
heatH2O(l). This is an endothermic process because the liquid water needs
heat to become a gas. The water uses heat from the body for this phase
change, thus heat and water are removed from the body.
e. When NaCl is added to water it dissolves and dissociates into two ions. The
dissolved ions are attracted to the solvent molecules of water through ion / dipole
forces. The forces of attraction between the ions and the water reduce the vapor
pressure of the system. Thus it takes more energy for the vapor pressure of the
system to equal the atmospheric pressure causing water to boil at a higher
temperature.
4) Carbon tetrachloride is a nonpolar compound (draw the Lewis dot structure to confirm).
Ammonium nitrate an ionic compound will not dissolve in polar solvent. 1-pentanol is a
polar molecular compound due to the presence of the hydroxyl (OH) group at the end of
the molecule. Pentane is a nonpolar compound. The nonpolar pentane will dissolve in
the nonpolar carbon tetrachloride solvent, while the polar 1-pentanol will not.
5) Ionic substances dissolve if the forces of attraction between the cations and the anions
are less that the force of attraction between the solvent and the ions. The forces of
attraction between the highly polar water molecule and the ions are strong enough to
dissociate/dissolve. The forces of attraction with the less polar ethanol are not strong
enough to pull the ions apart. This can be shown by drawing the Lewis dot structures
and labeling the dipoles.
6) Letter
a. Oil is a nonpolar and should float on top of the water. The polar salts are
dissolved in the water and must be isolated by removing the water.
b. Put on your safety goggles and pour the sample into a tared beaker. Record the
mass of the mixture.
c. Allow the sample to settle until most of the oil appears to have risen to the top.
Carefully decant the oil into a tared beaker and record the mass. Filtration
cannot be used her because the viscous oil will clog the filter paper preventing
both oil and the aqueous solution from passing through.
d. Place the remaining liquid inot a distillation apparatus and heat gently at first to
remove any volatile nonpolar material. Collect this material in a tared beaker and
record its mass.
e. Raise the temperature in the distillation apparatus until the water in the flask
distills. Stop distillation when it appears that all of the water has distilled. The
material remaining in the pot will not be completely dry. Use a wash bottle of
distilled water to transfer this material to an evaporating dish.
f. Heat the evaporating dish gently on a hot plate under the hood to remove any
remaining water. Let cool to room temperature and record the mass of the
solids.
g. The mass of water can be determined by subtraction of the mass of solids, oil
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7)
8)
9)
10)
and volatile polar material from the original mass. The mass of the oil will be
underestimated because it is difficult to remove all of the oil by decanting.
Solid A is most likely a metal. The delocalized “sea of electrons” result in high
conductivity. The strong electrostatic attraction between the delocalized electrons and
the metal cations, prevents the metal from dissolving in water. Solid B is most likely
ionic. Ionic solids are not conductive because their electrons not mobile – the electrons
are localized around their respective nuclei. Many ionic compounds are soluble in water,
and when they dissolve thy split up into anions and cations that are attracted to the polar
water molecules. The separation of charges leads to formation of a conductive solution.
Solid C is most likely network covalent, such as sand or diamond. The solids are not
particularly conductive because the electrons are localized in the covalent bonds
between atoms. They are not soluble in polar solvents because the polar molecules are
covalently bonded to themselves. Solid D is most likely molecular. Molecular
compounds do not conduct electricity because the electrons are localized in covalent
bonds between atoms. Nonpolar molecular compounds can be insoluble or slightly
soluble in water.
Polar molecular compounds will be soluble in water due to
intermolecular forces of attraction.
The lone pairs on the nitrogen and the oxygen atoms will be attracted to the slightly
positive hydrogen atoms of water, forming dipole-dipole attractions (specifically, hydrogen
bonds). The nonpolar CH3 (methyl) groups, will repel the polar water molecules. The
molecule may float on the water’s surface, with the hydrophobic portion facing up out of
the water and the hydrophilic portion facing down. Or it may organize in layers, with the
nonpolar portions facing each other, away from the water, and the polar portions facing
water.
Both molecules are able to make hydrogen bonds with their OH groups, so their boiling
points are relatively high. Because 1-propanol’s OH group is at the end of the carbon
chain, the molecule has more contact points available for London dispersion forces o
attraction and its boiling point is higher.
Both molecules are essentially nonpolar because of their very long carbon chains. The
primary intermolecular forces of attraction are London dispersion forces. Stearic acid is a
solid because it has a straight chain and so its molecules have many contact points and
strong London dispersion forces overall. The double bond in oleic acid creates a kink
(sp2 hybridization) in the chain so there are fewer contact points and weaker London
dispersion forces.
a. Sucrose, a molecular compound, does not dissociate in water while silver nitrate,
an ionic compound, does. The free floating ions in the silver nitrate solution
make it a good conductor.
b. Solid silver nitrate does not contain any mobile charged particles so it cannot
conduct electricity. Solid sodium metal atoms are held together by metallic
bonding – free floating metal cations in a sea of electrons – these mobile
electrons make solid sodium a good conductor.
c. In the liquid phase the ions of silver nitrate can move allowing it to conduct.
Sucrose does not contain charged particles in the liquid phase, so it cannot
conduct.
d. The ions in sulfuric acid have greater mobility when dissolved in water than in the
liquid phase. The solution whas a greate conductivity than the liquid.
11) Henry’s Law: S=kP. In the closed bottle the pressure of CO 2 is 5.0 atm so the
equilibrium concentration of CO2 is S = (5.0 atm) x (3.1 x 10-2 mol /L•atm). Once the
bottle is open, the pressure of CO2 is significantly reduced: S = (4 x 10-4 atm) x (3.1 x 102 mol /L•atm) and soft drink becomes flat.
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