11a - Synthesis of Alum

Synthesis of Alum

The instructor demonstrates how to synthesize alum—potassium aluminum sulfate dodecahydrate—from aluminum. Aluminum powder is used instead of foil to speed up the reaction.

Procedure and key observations

1. Measure the aluminum

   • Record the mass of a clean, dry reaction beaker: 51.9222 g.
   • Add aluminum and record the combined mass: 52.4066 g. The difference, 0.4844 g, is the mass of aluminum used to calculate the theoretical yield.
   • The instructor intended to use about 0.4 g of aluminum. The spoken estimate of the expected combined mass appears to be a slip; the recorded masses imply 0.4844 g of aluminum.

2. React the aluminum with potassium hydroxide

   • In a fume hood, add 20 mL of potassium hydroxide to the beaker.
   • The mixture bubbles as it reacts. The instructor says the reaction is complete when bubbling stops and the liquid becomes mostly blackish.
   • Use a fume hood because the reaction can generate hydrogen gas and a fine mist. Potassium hydroxide is not the limiting reactant.

3. Add sulfuric acid

   • Measure 7.5 mL of sulfuric acid separately, using a graduated cylinder or another suitable measuring device—not the markings on a beaker.
   • Once the aluminum has reacted, add the acid dropwise while stirring.
   • A solid forms as the alum is produced.

4. Dissolve and recrystallize the alum

   • Heat the mixture gently on a low-set hot plate until the crystals dissolve. Do not boil the solution.
   • The instructor notes that a little more sulfuric acid may help clear any remaining solid.
   • Cool the solution to form crystals. For the demonstration, it is cooled quickly in an ice bath; slower cooling over about a week would produce larger crystals.
   • Although the procedure calls for additional cooling time, the instructor proceeds early because crystals are already forming, noting that this could lower the yield.

5. Collect and rinse the crystals

   • Set up vacuum filtration with filter paper, moistened so it adheres to the funnel.
   • Pour the mixture through the funnel, keeping the alum crystals and discarding the liquid.
   • Rinse any remaining crystals from the beaker with cold ethanol. The instructor uses 95% ethanol, rather than the procedure’s requested 50% solution, and uses about 30 mL total over several rinses.
   • Pull air through the crystals with the vacuum for a few minutes to dry them.

6. Weigh the product and calculate yield

   • Record the mass of a clean, dry watch glass: 30.7327 g.
   • Transfer the alum crystals to the watch glass and weigh them. The recorded combined mass is 37.6111 g, giving an approximate actual alum yield of 6.8784 g by subtraction.
   • Calculate theoretical yield from the aluminum amount, then calculate percent yield:

   Percent yield = (actual yield ÷ theoretical yield) × 100%

   • The instructor observes that the product may still be slightly damp. Incomplete crystallization, crystals left behind during transfer, and insufficient drying can affect the measured yield.

Speakers and sources

• Chemistry instructor/narrator: Performs the demonstration and explains the procedure.
• Lab partner: Briefly mentioned as helping with a paper towel; no speech is included.
• Lab procedure: Referenced by the instructor as the source of the written steps.