5 Measure 25 mL of 4M H 2 SO 4 into a clean dry 50 mL graduated cylinder The

5 measure 25 ml of 4m h 2 so 4 into a clean dry 50 ml

This preview shows page 50 - 53 out of 140 pages.

5. Measure 25 mL of 4M H 2 SO 4 into a clean, dry 50 mL graduated cylinder. The clear solution is then made acidic by the slow addition of H 2 SO 4 . The acidification should be accompanied by continuous stirring. (CAUSTION: H 2 SO 4 IS TOXIC AND CORROSIVE. NEVER LEAVE HOT H 2 SO 4 SOLUTION UNATTENDED.) After the sulfuric acid has been added, the solution may contain large lumps of aluminum hydroxide. While constantly stirring it, this reaction mixture is then heated on a hot plate, gently, for about 5 minutes until all solid dissolves. 6. Use hot hand to remove the clear solution from the heat, and cool by first setting the beaker in tap water, then setting it an ice bath while gently stirring for about 10 minutes until granular crystals appear. Scratch the sides of the beaker with a glass rod to help stimulate crystallization. 7. Place a Filtervac adapter on top of the filter flask. Then place the Buchner funnel into the adapter. Attach the side arm of the filter flask to a vacuum with rubber tubing. 8. Weigh and record the mass of a 9-cm filter paper circle. Place the filter paper over the holes of the Buchner funnel. Wet the filter paper with DI water so that the filter paper adheres evenly to the surface of the funnel. Turn on the vacuum to make sure that the filtering apparatus is air tight. 9. While keeping vacuum on, transfer all crystals onto the filter paper with the help of a policeman (a glass stirring rod with rubber on a tip). Wash the crystals with 3 mL of a 95% alcohol mixture (in which alum is not very soluble). Gently dragging the policeman through damp crystals helps them dry. Continue vacuum until the crystals and filter paper appears dry. 50
10. Turn off the vacuum. Keeping the crystals on the filter paper. Move the filter paper (with the crystals on top) onto a watch glass. Weigh and record the combined mass of the alum crystals and filter paper. 11. Discard your synthesized alum and the filtrate in the filter flask into the proper containers provided by your instructor. Report Sheet ___________________ ____________ ______ Name Lab Partner Date Alum (Record masses to the nearest 0.0001 g) Ste p Object Trial 1 Trial 2 1 Mass of aluminum foil used 2 Mass of alum crystals and filter paper 3 Mass of filter paper Show Calculations: (Note: Some instructors will require this math in your lab notebook.) 4. Actual yield calculations: ______________ ____________ g 5. Theoretical yield calculations: ______________ ____________ g 6. Percent yield calculations: ______________ ____________ % Post Lab Questions: 1. List any sources or error in this experiment. Why might your percent yield be less than 100%? How could this have been improved? 51
2. Why was water a poor choice to rinse the Alum crystals with? 3. If your sample did not dry with the vacuum filtration, why didn’t we just put it in the oven to dry? 4. If a student needed to make 25.678 g alum and KOH and H 2 SO 4 are in excess, how much Al should the student start with knowing that they would only get an 80% yield of alum?

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture