Unformatted text preview: actly one
gram, but it is important to know exactly how much you do have. It is better to have slightly
more than one gram than slightly less because of significant figures. Place these pieces into a 50
mL beaker; add about 5 mL of water, and CAREFULLY add 4.0 mL of concentrated sulfuric
acid. Place the beaker on an iron ring with a wire mesh and lower the inverted funnel over it.
Heat gently with a Bunsen burner, bringing the solution to a gentle boil. Do not allow it to boil
too vigorously, and add water if the volume starts getting too low. Continue heating until you
are convinced that there is no more aluminum, but no more than 30 minutes.
Allow the solution to cool, adding water if necessary. Use gravity filtration to remove
undissolved paint. Put the filtrate in a clean 50 mL beaker, and place the beaker into an ice bath.
To the filtrate, slowly add 4.0 mL of 10. M KOH (aq) with stirring; be very careful to avoid
splashing or overheating during this step. Check to make sure the solution is still acidic to litmus
paper; if it is not acidic, add concentrated sulfuric acid dropwise until it is acidic.
Return the beaker to the iron ring, and give the solution a few minutes to warm up to
room temperature. Slowly apply heat until the solid dissolves. If the solution is boiling gently,
and there is still some solid in the beaker, add a little more water, as little as needed, until all of
the solid dissolves.
When the solid dissolves completely, remove the heat. Put the beaker (carefully) back
into the ice bath. If crystals do not begin forming in a few minutes, try scratching the bottom of
the beaker with a glass stirring rod. Allow the solution to continue to cool, while setting up a
vacuum filtration apparatus.
When the solution is cold, filter it using vacuum filtration. Wash the precipitate three
times with cold methanol. Allow the vacuum to continue to run for a few minutes to dry off as
much of the methanol as possible.
Weigh a watch glass and record the mass. When the crystals are cool, scrape them onto
the watch glass, and weigh it again. Dispose of the crystals as instructed to do so in class; your
professor may or may not want to collect them. Dakota State University Page 79 of 232 Experiment 4: Synthesis of a Compound General Chemistry I and II Lab Manual Calculations:
Balance the reactions of each step of the reaction:
(1) Dissolving the aluminum:
Al (s) + H2 SO4 (aq) à Al2 (SO4 )3 (aq) + H2 (g) (2) Neutralizing the excess acid:
H2 SO4 (aq) + KOH (aq) à K2SO4 (aq) + H2 O (l) Precipitate the produc t:
KAl(SO4 )2 (aq) + H2 O (l) à KAl(SO4 )2 .9H2 O (s) And balance the overall reaction:
Al (s) + H2 SO4 (aq) + KOH (aq) + H2 O (l) à KAl(SO4 )2 .9H2 O (s) + H2 (g) From the mass of the aluminum, calculate the theoretical yield of the Alum using standard
stoichiometric calculations. From the theoretical yield and the actual yield, calculate the percent
yield. Dakota State University Page 80 of 232 Experiment 4: Synthesis of a Compound General Chemistry I and II Lab Manual Observations:
Mass of Aluminum:
Observations: Mass of Watch Glass + Product:
Mass of Watch Glass:
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