Experiment 1 Chemy Sem 4

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    1. Objectives :

    To purify the naphthalene by recrystallization procedure and to determine the recovery

    percentage.

    2. Introductions:

    Recrystallization is an important method for the purification of solids. Recrystallization

    involves dissolving a solid in a solvent and crystallizing it again, taking the opportunity to

    discard impurities along the way. One normally chooses a solvent in which the solubility

    increases significantly with temperature.

    The solid is dissolved in a minimal amount of hot solvent, and the solution is filtered to

    remove insoluble impurities (e.g.other compounds, dust, etc). On cooling of the solution,

    the desired compound crystallizes, leaving soluble impurities in solution.

    Normally, one would like to recover the entire desired solid completely free of

    contaminants. Unfortunately, this is rarely possible. Usually, if most of the material is

    recovered, it is not very pure, and extremely pure material can be obtained only in low

    yield. The trick is to find the proper balance between yield and purity.

    3. Apparatus:

    a) Bunsen burner

    b) Stirrer rod

    c) Beaker

    d) Naphthalene

    e) Ice bath

    f) Erlenmeyer Flask

    g) Buchner funnel

    h) Filter paper

    i) Distilled water

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    4. Procedure:

    a) 2g of naphthalene is weigh and put into 120ml or 250 Erlenmeyer flask.

    b) 50ml of water is added into the flask and is stirred vigorously.

    c) Bunsen burner is used to heat the solution just to the boiling point and is stirredcontinuously until the naphthalene dissolves.

    d) The flask is placed in an ice bath for 5 minutes and is stirred occasionally.

    e) A properly fitted piece of filter paper is placed in Buchner funnel. All the holes are

    covered. The paper is wet thoroughly with deionized water from wash bottle and

    vanillin crystals are vacuum filtered. The flask is rinsed once or twice with 5 ml of

    ice-cooled water and this water is used to wash the crystals in the funnel.

    f) The crystals are empty onto a weighed piece of filter paper and then are placed

    in desk for drying

    5. Result and discussion

    a) Mass of naphthalene before recrystallization = 2.0 g

    b) Mass of filter paper. = 0.52 g

    c) Mass of naphthalene + filter paper after recrystallization = 2.0156 g

    d) Mass of crude naphthalene after recrystallization = 2.0156 0.52

    =1.50 g

    e) The percent recovery =

    X 100%

    = 75%

    f) The total lost = 100% - 75% = 25%.

    g) Why is the naphthalene solution cooled in ice bath before vacuum filtration?

    The naphthalene solution is placed in an ice bath to maximize the amount of solid

    that can precipitate from the solution. The recrystallized solid is then collected by

    vacuum filtration and washed with cold solvent.

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    h) How does the recrystallization procedure remove soluble impurities?

    Soluble impurities are the easiest to get rid off. When the solution is cooled down ,

    the product will crystallize first. The soluble impurities will be carried away during

    the filtration.

    i) What additional step should be added to this procedure to remove the impurities

    insoluble in solvent?

    Centrifugation step should be added. This step required to separate the

    recrystallized solid from the recrystallization solvent.

    j) Gravity and vacuum filtration separate insoluble solids from a liquid phase. The

    choices depend on conditions. Suggest criteria you would apply to choose between

    them?

    Gravity filtration is useful when removing minor amounts of solid impurities from

    solution. It can be done on a hot solution where the vacuum filtration cannot be done.

    k) Suggest any ways you can think of improve any parts of this experiment?

    Choosing an appropriate solvent is can improve the result that will be gained. The

    organic compound must have low solubility in the solvent at room temperature and

    moderate solubility at the boiling point. Ideally, the solubility of the impurities is

    greater than the organic compound at both high and low temperatures obviating the

    need for a hot filtration step.

    6. Conclusion

    The recovery percentage of naphthalene is 75%.


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