Materials and Equipment:
- Data collection system - Funnel
- Stainless steel temperature sensor - Dropper
- Ring stand with ring - Hot plate
- Clamp, utility - Bunsen Burner
- Clamp, buret - Clay Triangle
- Crucible with lid - 0.5 M Barium chloride (BaCl2), 30-mL
- Tongs - 0.1 M Silver nitrate (AgNo3), 5-mL
- Beaker, glass, 400-mL - 6 M Hydrochloric acid (HCl), 5-mL
- Beaker, glass, 250-mL - Unknown alkali sulfate, 0.53 g4
- Beaker (3), 25-mL - Filter Paper, Whatman Ashless, #42
- Beaker or flask, 400-mL - Rubber policeman and stirring rod
- Graduated cylinder, 100-mL - Watch glass, 100-mm
- Graduated cylinder, 10-mL - Distilled water, 100 mL
- Buret, 50 mL - Wash bottle with distilled water
Methods:
- Clean the 400-mL beaker, the stirring rod, rubber policeman, and 100-mm watch glass.
- Obtain a weighing dish containing a sample of unknown substance from your instructor.
- Measure the mas of the sample by difference of approximately 0.350 g or 0.800 g of unknown, as instructed by your teacher, into the 400-mL beaker.
- Add 50 mL of distilled water to the beaker, and dissolve the sample with gentle swirling.
- Slowly add 5 mL of 6 M HCl to the solution in the beaker.
- Continue adding distilled water to the solution until there is about 250 mL of solution in the beaker.
- Cover the beaker with the 100-mm watch glass, and store it in a safe place until you are ready to continue with the determination.
- Begin forming the precipitate by starting a new experiment on the data collection system.
- Attach the buret to the ring stand with the buret clamp.
- Connect the stainless steel temperature sensor to the data collection system.
- Heat the beaker with the solution on the hot plate to about 90° C. Do not allow the solution to boil because some of the solution might be lost through splattering.
- Immerse the temperature sensor in the solution (do not allow it to touch the beaker).
- Set the data collection system to monitor live data without recording.
- Once the solution has reached 90°C, begin adding, drop by drop, the 0.5 M BaCl2 solution from the buret into the beaker while gently swirling the solution with the rubber policeman.
- After adding 25 to 35 mL of BaCl2 solution, stop the addition and allow the precipitate to settle.
- Once the precipitate has settled, test for completeness of the precipitation by adding 1 to 2 drops of BaCl2. If you see any sign of precipitation, add 5 more millimeters of BaCl2.
- Rinse the rubber policeman over the beaker with the solution with a few drops of deionized water. Cover the beaker with a watch glass and continue heating at 90° C for one hour (the precipitate should be coarse and the supernatant should be clear).
- Fold a piece of Whatman Ashless #42 filter paper in half and then fold it in half again.
- Hold three of the four parts of the filter paper on one side, and then pull out the fourth part of the filter paper to form a cone.
- Place the cone-shaped filter paper into the funnel.
- Wet the filter paper with a few drops of distilled water so that the paper sticks to the wall of the funnel.
- Support the funnel in the ring mounted on a ring stand over a 400-mL beaker or flask, to hold all of the supernatant.
- Heat 100 mL of distilled water, in the 250-mL beaker, to about 80°C for rinsing the precipitate.
- Pour the mixture with the percipitate through the funnel with the filter paper.
- Remove any remaining precipitate from the beaker using the rubber policeman and small washes of hot distilled water.
- Wash the material in the funnel with three 5-mL portions of the hot, distilled water. Collect the washes separately in the small 25-mL beakers.
- Add two drops of the AgNO3 solution to the last wash. If any cloudy white precipitate is observed, the precipitate still contains chloride ions and must be washed with hot distilled water a fourth time in a rinsed beaker.
- Heat the crucible and lid on the clay triangle over a bunsen burner with a gentle flame for 5 minutes.
- Once the bottom of the crucible has become red hot, increase the flame and continue heating for 10 to 12 minutes.
- Turn off the bunsen burner and allow the crucible to cool.
- Measure and record the mass of the "fired" crucible. Repeat steps 28-30 until two readings for the mass are within 10 mg of each other.
- Remove the filter paper containing the precipitate from the funnel. Carefully fold the filter paper so that the precipitate is trapped inside and the final shape is small enough to fit into the crucible.
- Heat the crucible on a wire triangle on a ring stand over a bunsen burner (use a small flame so that the filter paper does not burst into flames). Gradually increase the size of the flame so that the entire crucible is heated.
- Continue heating until all of the carbon residue from the filter paper has been removed. Then continue heating the crucible for an additional 10 minutes.
- Allow the crucible to cool. Measure and record the mass of the crucible, lid, and precipitate.
- Repeat steps 33-35 until two successive mass measurements are within 10 mg of each other.
- Clean up according to your teacher's instructions.