UCSB CHEM 6AL FINAL EXAM 2026 QUESTIONS WITH COMPLETE
SOLUTIONS.
How does the vapor composition compare to the liquid composition in a
50:50 mix of two liquids? - ANS.... -if we have pentane and hexane, vapor
would not be 50:50, it would be more pentane because pentane has lower
BP.
Temperature composition diagram - ANS.... -Lower curve = liquid line
Upper curve = vapor line
Can determine the composition of both liquid and gas phases at certain
temperatures as well as the boiling points of certain compositions
Ex: at .8 (point B) decane boiling point is ~100 degrees C and the vapor
composition (at point C) is .5 decane
Intermolecular forces and how they affect physical properties /purification
techniques: dipole-dipole, electrostatic, hydrogen, ion, london, - ANS.... -
*dipole-dipole*: polar covalent molecules such as aldehydes and ketones.
Positive and negative end. Polar molecules must be close together for
force to be significant. Weaker than ion-dipole forces; increase with polarity
of molecule.
*electrostatic*: occur between charged species, cations and anions, and
are responsible for very high MP and BP of ionic compounds and metals
*hydrogen bonding*: hydrogen atom in a polar bond (NOF). Considered a
dipole-dipole interaction. Quite polar.
,*ion-dipole*: interaction betwween a charged ion and apolar molecule.
Cations are attracted to negative end of dipole, and anions are attracted to
positive end.
*london dispersion*: dependent on surface area and polarizability of the
surface of the molecule. Only types of forces NON-polar covalent
molecules can experience. Result from the movement of electrons in the
molecule, which generates temporary positive and negative regions in the
molecule.
Melting point and melting point range - ANS.... -solid has ordered crystal
lattice structure. Heat makes this structure move enough to the point where
it becomes a liquid. The melting point is how much energy it takes to make
this lattice structre move. Impurities disrupt the crystal lattice, so impurities
make a structure easier to melt. MP also broadens when a compound is
impure. If there is water in the solid, it is impure. Solid will stick to side of
capillary if still wet.
Best way to determine MP range if you dont know the MP range:
Take a quick measurement to get a rough estimate, then take a second,
more careful measurement.
Limiting reagents and percent yields - ANS.... -*limiting reagents*: the
compound that runs out first. When limiting reagent runs out, reaction can
no longer proceed and other compound is considered in excess. Tell which
is limiting reagent by converting grams or mg used to moles using molar
mass. When you have moles, use the stoichimetric ratios to see which is
the smaller amount of moles (mole of reagent under mole produced of
product); this is your limiting reagent. Could also compare mole ratios
between reagents to see how much you would need of the other reagent to
make reaction go to completion. Use limiting reagent to see how much
product we will get.
,*percent yield*: actual yield/theoretical yield X 100. If you expect 10 g but
only get 8.4, your % yield is 84%. Get theoretical yield by taking limiting
reagent and seeing how much product can be made by LR. Convert g to
mol of LR, then convert mol of LR to mol of product; use molar ratio for this.
Then convert the mol of product by multiplying molar mass of product.
Distillation: ideal liquids/Raoults law - ANS.... -*ideal liquid*: a liquid that in
incompressible (density is constant), irrotational (no turbulence, flow is
smooth), and nonviscous (fulid has no internal friction). They do not exist in
nature. Does not change composition by mixing. It obeys Raoult's law for
all compisitions.
*Rauoults Law*: shown in picture. Total pressure can be found by adding
up partial pressure; partial pressure can be found with Raoult's law.
To find mol fraction (Xa), divide mols of A over total moles of solution.
Assumption of raults law:
1) the liquid is ideal
The contribution of each liquid phase can be found by determining the mole
fraction of each in the vapor phase:
(gama)A = PA/Ptotal
Example: what is the partial pressure of etoh (Po=45 kpa, MW= 46.07
g/mol) if 3.0 g of it is mixe with 3.0 g of unknown (MW= 32.04)? - ANS.... -
18.5
, A compound with high IMF will have a ____ BP than a compound with low
IMF; Imagine a solution of water boiling in a round flask. The vapors above
this liquid are ______ the liquid themselves. - ANS.... -higher; the same
temp as
T or F: higher vp means that a liquid is harder to turn into a vapor - ANS....
-F its easier!
T or F: molecules with lower IMF exert lower vapor pressures - ANS.... -F
they prodcuce higher vps!
T or F: compounds with a high vp have a low bp - ANS.... -T :)
Which substance in the distillation mixture will make up more of the vapor
composition - ANS.... -the substance with the weaker intramolecular forces,
lower boiling point, and higher vapor pressure
Simple vs fractional distillation - ANS.... -*simple*: used to purify almost
already pure compounds, to separate liquid from solid impurities, and to
remove volatile (readily evaporating) solvents. Few cycles, heat source to
heat liquid. Have boiling stones in liquid. Vapors travel up and condense
back down into different tube. Works well if there is a large difference in BP
(60-70 degrees C). Only does one evaporation/ condenstion cycle). How
can we improve this? By using more theoretical plates (each TP represents
one evap/conden cycle). Do this by using fractional distillation
*fractional distillation*: fractionating columns increase Theoretical Plates.
Theoretical plates= surface area where vaporization/ condensation can
potential occur
SOLUTIONS.
How does the vapor composition compare to the liquid composition in a
50:50 mix of two liquids? - ANS.... -if we have pentane and hexane, vapor
would not be 50:50, it would be more pentane because pentane has lower
BP.
Temperature composition diagram - ANS.... -Lower curve = liquid line
Upper curve = vapor line
Can determine the composition of both liquid and gas phases at certain
temperatures as well as the boiling points of certain compositions
Ex: at .8 (point B) decane boiling point is ~100 degrees C and the vapor
composition (at point C) is .5 decane
Intermolecular forces and how they affect physical properties /purification
techniques: dipole-dipole, electrostatic, hydrogen, ion, london, - ANS.... -
*dipole-dipole*: polar covalent molecules such as aldehydes and ketones.
Positive and negative end. Polar molecules must be close together for
force to be significant. Weaker than ion-dipole forces; increase with polarity
of molecule.
*electrostatic*: occur between charged species, cations and anions, and
are responsible for very high MP and BP of ionic compounds and metals
*hydrogen bonding*: hydrogen atom in a polar bond (NOF). Considered a
dipole-dipole interaction. Quite polar.
,*ion-dipole*: interaction betwween a charged ion and apolar molecule.
Cations are attracted to negative end of dipole, and anions are attracted to
positive end.
*london dispersion*: dependent on surface area and polarizability of the
surface of the molecule. Only types of forces NON-polar covalent
molecules can experience. Result from the movement of electrons in the
molecule, which generates temporary positive and negative regions in the
molecule.
Melting point and melting point range - ANS.... -solid has ordered crystal
lattice structure. Heat makes this structure move enough to the point where
it becomes a liquid. The melting point is how much energy it takes to make
this lattice structre move. Impurities disrupt the crystal lattice, so impurities
make a structure easier to melt. MP also broadens when a compound is
impure. If there is water in the solid, it is impure. Solid will stick to side of
capillary if still wet.
Best way to determine MP range if you dont know the MP range:
Take a quick measurement to get a rough estimate, then take a second,
more careful measurement.
Limiting reagents and percent yields - ANS.... -*limiting reagents*: the
compound that runs out first. When limiting reagent runs out, reaction can
no longer proceed and other compound is considered in excess. Tell which
is limiting reagent by converting grams or mg used to moles using molar
mass. When you have moles, use the stoichimetric ratios to see which is
the smaller amount of moles (mole of reagent under mole produced of
product); this is your limiting reagent. Could also compare mole ratios
between reagents to see how much you would need of the other reagent to
make reaction go to completion. Use limiting reagent to see how much
product we will get.
,*percent yield*: actual yield/theoretical yield X 100. If you expect 10 g but
only get 8.4, your % yield is 84%. Get theoretical yield by taking limiting
reagent and seeing how much product can be made by LR. Convert g to
mol of LR, then convert mol of LR to mol of product; use molar ratio for this.
Then convert the mol of product by multiplying molar mass of product.
Distillation: ideal liquids/Raoults law - ANS.... -*ideal liquid*: a liquid that in
incompressible (density is constant), irrotational (no turbulence, flow is
smooth), and nonviscous (fulid has no internal friction). They do not exist in
nature. Does not change composition by mixing. It obeys Raoult's law for
all compisitions.
*Rauoults Law*: shown in picture. Total pressure can be found by adding
up partial pressure; partial pressure can be found with Raoult's law.
To find mol fraction (Xa), divide mols of A over total moles of solution.
Assumption of raults law:
1) the liquid is ideal
The contribution of each liquid phase can be found by determining the mole
fraction of each in the vapor phase:
(gama)A = PA/Ptotal
Example: what is the partial pressure of etoh (Po=45 kpa, MW= 46.07
g/mol) if 3.0 g of it is mixe with 3.0 g of unknown (MW= 32.04)? - ANS.... -
18.5
, A compound with high IMF will have a ____ BP than a compound with low
IMF; Imagine a solution of water boiling in a round flask. The vapors above
this liquid are ______ the liquid themselves. - ANS.... -higher; the same
temp as
T or F: higher vp means that a liquid is harder to turn into a vapor - ANS....
-F its easier!
T or F: molecules with lower IMF exert lower vapor pressures - ANS.... -F
they prodcuce higher vps!
T or F: compounds with a high vp have a low bp - ANS.... -T :)
Which substance in the distillation mixture will make up more of the vapor
composition - ANS.... -the substance with the weaker intramolecular forces,
lower boiling point, and higher vapor pressure
Simple vs fractional distillation - ANS.... -*simple*: used to purify almost
already pure compounds, to separate liquid from solid impurities, and to
remove volatile (readily evaporating) solvents. Few cycles, heat source to
heat liquid. Have boiling stones in liquid. Vapors travel up and condense
back down into different tube. Works well if there is a large difference in BP
(60-70 degrees C). Only does one evaporation/ condenstion cycle). How
can we improve this? By using more theoretical plates (each TP represents
one evap/conden cycle). Do this by using fractional distillation
*fractional distillation*: fractionating columns increase Theoretical Plates.
Theoretical plates= surface area where vaporization/ condensation can
potential occur
