CHM 205 – Final Exam Review – Questions and Solutions – 2025/2026
Why would you use a filtering pipet? - ANS✔✔ Used to remove unwanted solid from a liquid
-When you want the liquid, cotton plug stops solid
Why would you use a filter-tip pipet? - ANS✔✔ -to remove solids from very small amounts of
liquid by leaving solid behind (as opposed to trapping it)
-when you want the solid
Why (in exp. 0) do you rinse w/ 1 mL of methanol and 1 mL of hexane? - ANS✔✔ -methanol
dissolves and hhexane has a low bp to evaporate liquid
What m.p. indicates purity? - ANS✔✔ -a narrow melting point range
-an impurity lowers and broadens m.p.
-to lower freezing point, add impurity as well
-an intensive property
What does mixed melting point do? - ANS✔✔ -If the unknown and known are the same, then
there is no change in melting point range
- if not, then the range will react like an impurity was added
Melting point range - ANS✔✔ temperature of first liquid- temperature of completely melted
When using the mel-temp, why shouldn't you heat faster than 1-2 ˚C? - ANS✔✔ -can result in
misread temperature due to crystal lagging
,What does it mean if the solution in the mel-temp turned from white to brown? - ANS✔✔ -
Decomposition occurred
What happened if the solution in the mel-temp disappeared? - ANS✔✔ -sublimation occurred,
seal capillary
What is the literature m.p. value for ferrocene? - ANS✔✔ -173-174˚C
What is the literature m.p. value for adipic acid? - ANS✔✔ -151-154˚C
What is the literature m.p. value for acetyl ferrocene? - ANS✔✔ -81-86˚C
What is the literature m.p. value for citric acid? - ANS✔✔ -153-159˚C
How do you calculate the number of drops from a given mL? - ANS✔✔ -x mL x # drops/ mL = x
drops
How do you calculate the number of drops from a given weight (g)? - ANS✔✔ - X g x cm^3/g x #
drops/mL = x drops
How do you calculate the # g from a given number of drops? - ANS✔✔ - # drops x 1 mL/ 28
drops = # mL x g/mL= # g
max of heat dial for a hot plate? - ANS✔✔ 4-390 degrees celsius
Why should you always add boiling chips before heating them to their boiling point? - ANS✔✔ -
to prevent bumping and local overheating (decreased evaporation)
, Why should you try to dissolve a compound in as little solvent as possible? (during
recrystallization) - ANS✔✔ -to increase compound crystallization from increased concentration
How are insoluble impurities removed? - ANS✔✔ -from hot gravity filtration
How are soluble colored impurities removed? - ANS✔✔ -from charcoal and subsequent HGF
why should you allow recrystallization to occur slowly? - ANS✔✔ to obtain a more pure
compound
explain what the colors of drierite mean. - ANS✔✔ blue: fresh and ready to use
pink: used; already contains water
Why should you never add charcoal (or an solid substance) to a boiling solution? - ANS✔✔ -it
could boil over
why do you scratch the flask? (in macroscale recrystallization) - ANS✔✔ -to create a surface for
nucleation
After macroscale recrystallization, what are two ways you can get more recovery? - ANS✔✔ -
cool further for more recovery
-evaporate for a higher concentration
What is the purpose of CaSO4? - ANS✔✔ -to dry solution/ absorb moisture
What is the solvent/solute used in the macroscale recrystallization? - ANS✔✔ -water and
organic unknown
Why would you use a filtering pipet? - ANS✔✔ Used to remove unwanted solid from a liquid
-When you want the liquid, cotton plug stops solid
Why would you use a filter-tip pipet? - ANS✔✔ -to remove solids from very small amounts of
liquid by leaving solid behind (as opposed to trapping it)
-when you want the solid
Why (in exp. 0) do you rinse w/ 1 mL of methanol and 1 mL of hexane? - ANS✔✔ -methanol
dissolves and hhexane has a low bp to evaporate liquid
What m.p. indicates purity? - ANS✔✔ -a narrow melting point range
-an impurity lowers and broadens m.p.
-to lower freezing point, add impurity as well
-an intensive property
What does mixed melting point do? - ANS✔✔ -If the unknown and known are the same, then
there is no change in melting point range
- if not, then the range will react like an impurity was added
Melting point range - ANS✔✔ temperature of first liquid- temperature of completely melted
When using the mel-temp, why shouldn't you heat faster than 1-2 ˚C? - ANS✔✔ -can result in
misread temperature due to crystal lagging
,What does it mean if the solution in the mel-temp turned from white to brown? - ANS✔✔ -
Decomposition occurred
What happened if the solution in the mel-temp disappeared? - ANS✔✔ -sublimation occurred,
seal capillary
What is the literature m.p. value for ferrocene? - ANS✔✔ -173-174˚C
What is the literature m.p. value for adipic acid? - ANS✔✔ -151-154˚C
What is the literature m.p. value for acetyl ferrocene? - ANS✔✔ -81-86˚C
What is the literature m.p. value for citric acid? - ANS✔✔ -153-159˚C
How do you calculate the number of drops from a given mL? - ANS✔✔ -x mL x # drops/ mL = x
drops
How do you calculate the number of drops from a given weight (g)? - ANS✔✔ - X g x cm^3/g x #
drops/mL = x drops
How do you calculate the # g from a given number of drops? - ANS✔✔ - # drops x 1 mL/ 28
drops = # mL x g/mL= # g
max of heat dial for a hot plate? - ANS✔✔ 4-390 degrees celsius
Why should you always add boiling chips before heating them to their boiling point? - ANS✔✔ -
to prevent bumping and local overheating (decreased evaporation)
, Why should you try to dissolve a compound in as little solvent as possible? (during
recrystallization) - ANS✔✔ -to increase compound crystallization from increased concentration
How are insoluble impurities removed? - ANS✔✔ -from hot gravity filtration
How are soluble colored impurities removed? - ANS✔✔ -from charcoal and subsequent HGF
why should you allow recrystallization to occur slowly? - ANS✔✔ to obtain a more pure
compound
explain what the colors of drierite mean. - ANS✔✔ blue: fresh and ready to use
pink: used; already contains water
Why should you never add charcoal (or an solid substance) to a boiling solution? - ANS✔✔ -it
could boil over
why do you scratch the flask? (in macroscale recrystallization) - ANS✔✔ -to create a surface for
nucleation
After macroscale recrystallization, what are two ways you can get more recovery? - ANS✔✔ -
cool further for more recovery
-evaporate for a higher concentration
What is the purpose of CaSO4? - ANS✔✔ -to dry solution/ absorb moisture
What is the solvent/solute used in the macroscale recrystallization? - ANS✔✔ -water and
organic unknown
