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Things You'll Need
Instructions
Put on your gloves, goggles and lab coat.
Place the 0.5 grams of impure benzoic acid in a 50-mL Erlenmeyer flask.
Set up your hot plate and plug it in. Pour the 200 mL of deionized water into a beaker, add a few boiling chips or stones and place the beaker on the hot plate. Heat the water up until it reaches a gentle boil.
Using your Pasteur pipette, add small increments of hot water to the impure benzoic acid sample while swirling the flask until it dissolves. Don't use more hot water than you need. The less hot water you need to dissolve the sample, the better.
Allow the solution you've just made to cool slightly and add a few charcoal pellets and boiling chips. Never add charcoal pellets or boiling chips to a solution that's already boiling -- you could cause it to boil over.
Place the flask on the hot plate and bring it back to a boil.
Take your piece of filter paper and fold it into a wedge so it's pleated like a coffee filter. Unfold it and place it in the glass funnel. Insert the glass funnel into the other Erlenmeyer flask.
Take your Pasteur pipette and damp the paper filter with some hot water from the beaker, but make sure the filter paper doesn't tear. Wet the funnel to preheat it. The funnel should be hot or at least warm so the benzoic acid doesn't start to crystallize in the funnel.
Pick up the flask containing the dissolved benzoic acid using tongs. Don't try to handle it with your bare hands if it's hot. If you can't tell whether a piece of glassware is hot, just hold your hand a short distance from it to see whether it's radiating heat.
Pour the dissolved benzoic acid through the filter paper. This step should filter out any insoluble impurities (impurities that absolutely will not dissolve in water). It also removes the charcoal and boiling chips.
Turn off the hot plate. Remove the funnel and filter paper.
Watch the benzoic acid solution as it cools. It should start to form crystals quite rapidly. If it doesn't start to form crystals, you can add a single crystal of impure sample as a seed crystal, or try gently scraping the base inside of the flask with a spatula.
Allow the solution at least 20-30 minutes for crystal formation. Then place it in an ice bath for 10 minutes.
Connect one of the two hoses from the vacuum outlet to the sidearm of the first filter flask. Insert the vacuum adapter into the mouth of this flask and connect the second hose to the vacuum adapter at one end and the sidearm of the second filter flask on the other.
Place the Buchner funnel in the mouth of the second filter flask. You have now set up a vacuum filtration system. You may want to clamp the filter flasks to a ringstand to keep them in place.
Wet the filter paper with a little cold water. Turn on the vacuum outlet gently so you have vacuum.
Pour the cold crystal-containing solution from the Erlenmeyer flask into the Buchner funnel. The crystals will be trapped by the filter paper, while the solution will be sucked through into the filter flask. Water-soluble impurities will be removed by this step.
Rinse the inside of your Erlenmeyer flask with a little cold water and pour the rinse into the funnel. If needed, remove any crystals from the inside of the flask with a spatula and place these in the funnel also.
Sprinkle the crystals with cold water from your Pasteur pipette to "wash" them and remove any remaining water-soluble impurities.
Turn off the vacuum. Transfer the crystals from the filter paper to a more suitable receptacle of your choice, such as a small beaker, with the aid of a spatula.
Place the small beaker with the crystals in it in your desiccator and leave it there overnight so the crystals are dry.
Clean your workspace.