Answer to Question #76120 in General Chemistry for Vy

Question

1. Why does reducing the volume of your solution (to 50 mL) increase your yield of solid alum?
2. Why should the caustic potash be added to the metal foil in the fume hood instead of out in the lab?
3. What specific impurities are removed when you wash the final product? Please list them.
4. Why is the final product washed with 50% ethanol solution instead of water?

Answer 1

1. This is due to the lower hydrolysis level of aluminium salt in a more concentrated solution.
2. Hydrogen gas is given off when aluminium metal is reacted with potassium hydroxide solutions. Hydrogen is extremely flammable. That’s why the caustic potash should be added to the metal foil in the fume hood instead of out in the lab.
3. It can be some sulfuric acid, which left after the reaction.
4. The final product washed with 50% ethanol solution instead of water to avoid dissolving much crystals and because a lot of inorganic impurities can be removed by an organic solvent wash.