Chem 331 Lab Practical 1 Questions
and All Correct Answers.
What are the four exposure routes by which harmful chemicals can enter the body? - Answer
Inhalation, injection, absorption and ingestion
You should never heat a _____ ______ with a burner flame - Answer graduated cylinder
How to you properly dilute concentrated acids and bases? - Answer By pouring the reagent
into water while stirring carefully and constantly. Never pour water into concentrated acid (add
acid) and never add concentrated acid to concentrated base of vice versa.
PPE - Answer personal protective equipment
MSDS - Answer Material Safety Data Sheet
PEL - Answer Permissible Exposure Limit
TLV - Answer Threshold Limit Value (bp, mp, vp)
Mixed melting point - Answer The identity of two samples that have the same or similar
melting point can be determined by taking mixed melting point
What does it mean when the melting point of two substances is depressed? - Answer They
are not identical
Oneset point - Answer Temperature at which the first drop of liquid appears among the
crystals
Clear point - Answer temperature at which the entire mass of solid turns into clear liquid
Eutectic point - Answer Lowest melting point of an A/B mixture
How do you load a melting point capillary? What are the consequences of doing this
improperly? - Answer It must be fully dry, homogenous and in powered form. Using too
much sample or not packing it correctly can cause an unusually wide mp range, air pockets can
cause uneven heating, and the substance should not melt at a higher temperature than its
melting point
, Purity of a melting point can be checked by - Answer seeing if the mp width is less than 2.0
which shows purity, but any larger than that it's impure. IF the melting point has decreased
compare to the lit. value that indicates an impurity
Effect of sample height on mp data is - Answer increased height causes the mp range to be
wider and lower the onset point. It takes longer to melt
Melting point - Answer the temperature at which the first crystal just starts to melt until the
temperature at which the last crystal disappears
A student prepares a sample of impure benzoic acid for melting point analysis (118-120). She
sets the DigiMelt start temperature to 115.0°C, and presses the start button. The sample begins
melting immediately, and the student records the melting point range of the sample as 115.1 -
115.4°C. Did she accurately record the melting point of this sample? Explain. - Answer No,
her onset point was much lower than the start temperature she had set. The onset point is
<115.1, so she started at a temperature that was too high.
Why does re-solidified chemicals give different melting points? - Answer Heat causes
substances to change chemically, this lowers and widens the mp range. This decomposition
could explain this discrepancy.
Dissolved materials have _______ solubility at lower temperatures and will _____ from the
solution as it's cooled - Answer decreased; separate
Purposes of recrystallization - Answer Purify crude solids, separate compounds based on
differences in solubility
Crystallization definition is - Answer Crystal growth is slow and selective
Precipitation - Answer Crystal growth is rapid and nonselective
Ideal recrystallization solvents - Answer can dissolve the entire compound at the boiling
point of the solvent, dissolves little of compound when solvent is at room temperature
What is the effect of too much solvent in the recrystallization lab? - Answer Too much causes
the percent recovery to be low or no crystals will form
Methods to remove excess solvent for recrystallization? - Answer Evaporation by heating
using air stream or nitrogen
and All Correct Answers.
What are the four exposure routes by which harmful chemicals can enter the body? - Answer
Inhalation, injection, absorption and ingestion
You should never heat a _____ ______ with a burner flame - Answer graduated cylinder
How to you properly dilute concentrated acids and bases? - Answer By pouring the reagent
into water while stirring carefully and constantly. Never pour water into concentrated acid (add
acid) and never add concentrated acid to concentrated base of vice versa.
PPE - Answer personal protective equipment
MSDS - Answer Material Safety Data Sheet
PEL - Answer Permissible Exposure Limit
TLV - Answer Threshold Limit Value (bp, mp, vp)
Mixed melting point - Answer The identity of two samples that have the same or similar
melting point can be determined by taking mixed melting point
What does it mean when the melting point of two substances is depressed? - Answer They
are not identical
Oneset point - Answer Temperature at which the first drop of liquid appears among the
crystals
Clear point - Answer temperature at which the entire mass of solid turns into clear liquid
Eutectic point - Answer Lowest melting point of an A/B mixture
How do you load a melting point capillary? What are the consequences of doing this
improperly? - Answer It must be fully dry, homogenous and in powered form. Using too
much sample or not packing it correctly can cause an unusually wide mp range, air pockets can
cause uneven heating, and the substance should not melt at a higher temperature than its
melting point
, Purity of a melting point can be checked by - Answer seeing if the mp width is less than 2.0
which shows purity, but any larger than that it's impure. IF the melting point has decreased
compare to the lit. value that indicates an impurity
Effect of sample height on mp data is - Answer increased height causes the mp range to be
wider and lower the onset point. It takes longer to melt
Melting point - Answer the temperature at which the first crystal just starts to melt until the
temperature at which the last crystal disappears
A student prepares a sample of impure benzoic acid for melting point analysis (118-120). She
sets the DigiMelt start temperature to 115.0°C, and presses the start button. The sample begins
melting immediately, and the student records the melting point range of the sample as 115.1 -
115.4°C. Did she accurately record the melting point of this sample? Explain. - Answer No,
her onset point was much lower than the start temperature she had set. The onset point is
<115.1, so she started at a temperature that was too high.
Why does re-solidified chemicals give different melting points? - Answer Heat causes
substances to change chemically, this lowers and widens the mp range. This decomposition
could explain this discrepancy.
Dissolved materials have _______ solubility at lower temperatures and will _____ from the
solution as it's cooled - Answer decreased; separate
Purposes of recrystallization - Answer Purify crude solids, separate compounds based on
differences in solubility
Crystallization definition is - Answer Crystal growth is slow and selective
Precipitation - Answer Crystal growth is rapid and nonselective
Ideal recrystallization solvents - Answer can dissolve the entire compound at the boiling
point of the solvent, dissolves little of compound when solvent is at room temperature
What is the effect of too much solvent in the recrystallization lab? - Answer Too much causes
the percent recovery to be low or no crystals will form
Methods to remove excess solvent for recrystallization? - Answer Evaporation by heating
using air stream or nitrogen
