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ochem lab final review

ochem lab final review - 1 Recrystallization Background We...

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1. Recrystallization Background: We allowed an unknown compound to be dissolved into an organic solvent (which in this case was water.) As the temperature of the organic solvent rose, more of the unknown compound dissolved. At or near the boiling point, most of the unknown compound had dissolved. We then allowed the solvent to cool, and in this process, less of the compound remained dissolved in the organic solvent. The compound, instead of remaining dissolved in the organic solvent, crystallized out of the organic solvent. Once obtaining the crystals, we measured the melting point of the compound through mixed melting point to determine the identity of our unknown compound. We do recrystallization to purify organic compounds!!!! We take a compound with impurities, do this experiment, and get our pure compound. We did two filtrations in this lab: We did a hot filtration with a powder funnel to remove any insoluble impurities; we also performed a vacuum filtration to isolate the crystals that had formed. There are four characteristics of a good recrystallizing solvent: 1. At room temperature, the solvent should only dissolve the solute slightly. 2. At boiling point, the solvent should dissolve most of the solute 3. Solvent SHOULD NOT react with solute 4. Solute should be volatile enough to be removed from solvent. There are also 3 methods to use to produce crystals after submerging flask in ice bath: 1. Scrape side of flask; induces crystals 2. Put a sample of the desired compound in flask—induces crystallization 3. Heat the solvent and allow some of the solvent to evaporate; repeat ice bath. Here, you had too much solvent. Melting point: Time when solid phase is in equilibrium with liquid phase. BIG POINT: Having impurities in your compound will lower the melting point and also these impurities will broaden your melting point range. A narrow melting range is less than 3 degrees Celsius; a broad melting range is more than 3 degrees Celsius. MIXED MELTING POINT: Here we mix our unknown compound with a compound that we know the melting point of. If the two compounds are the same, you would expect the melting point to be the same as the one you have for the known. Percent Recovery: (weight recovered/original weight of sample) * 100%. If you receive a percent recovery that is greater than 100%, this is probably due to the fact that you did not allow the crystals to dry properly before weighing them. And since water has weight, you may end up going over the original weight of the sample. BIG POINT: As the temperature of the solvent increased, the amount of solute dissolved in the solvent increased; as the temperature of the solvent decreased, the amount of the solute dissolved in the solvent also decreased .
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II. Simple and Fractional Distillation Background: We know that distillation enables us to separate components of a liquid mixture. So, in this experiment, we wanted to separate a water and methanol mixture. We performed simple and fractional distillations to see which method would be best to separate the mixture. We heated our liquid mixture until it
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