Lab3_Acid_IDa - Chem 223 Identification of an Unknown...

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1 Chemistry 223 Identification of Weak Acid Chem 223 Identification of an Unknown Organic Acid Understanding weak acids and weak bases is a central part of quantitative analysis. The equilibria involved are important in all aqueous chemistry (thus, biochemistry), and the methods to study them help develop your critical thinking skills. Furthermore, to get good results, you have to carefully deal with measurement precision, data reduction, and chemical handling. You also must multi task. If you stare at boiled water while it cools, rather than working on other tasks, you will not finish this lab. In Week One of this lab, you will: Make concentrated NaOH Make carbonate-free water Dilute NaOH to leave it carbonate free Dry potassium hydrogen phthalate (KHP) to use as a primary standard Weigh 4 aliquots of KHP Titrate KHP with NaOH to standardize the NaOH Obtain an unknown weak acid Dry the unknown In Week Two, you will: Weigh 4 aliquots of unknown weak acid Titrate the weak acid with standard NaOH, monitoring each titration with a pH meter From the shape of the titration curve, determine the equivalent weight of the acid From the shape of the titration curve, determine the pK a (or pK a s) of the acid By comparison to published data, determine the identity of your unknown In light of what you have determined, specify pH's where your unknown could be used to make a buffer, and answer questions about what would be different if the unknowns were weak bases. How many times on CSI have you seen detectives encounter "an unknown white powder?" The powder could be sugar, artificial sweetener, an illegal substance, or one of the unknowns from this lab. While you can be sure we will only give you a weak acid unknown (there goes the suspense … ), you can experience the rush of solving a problem and knowing you've "nailed it." Instructions for Week One A. Make Concentrated NaOH Prepare some 50% by weight NaOH. Procedure: weigh out approximately 25 g of NaOH (this can not be done accurately; NaOH is hygroscopic. Getting it close on a top loading or triple beam balance is Good Enough). Add as many mL of distilled water as the mass of NaOH (so if you weigh 24 g of NaOH, use 24 mL water). Store in a tightly sealed 50 mL: (or 100 mL) plastic bottle. Swirl well to mix and dissolve the NaOH, then let settle so the Na 2 CO 3 settles out. Once the settling starts, DO NOT MOVE THE BOTTLE.
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2 Chemistry 223 Identification of Weak Acid Otherwise, it becomes necessary to filter the NaOH! Safety : if you get NaOH solution on your hands, it will feel slippery. If this happens, immediately rinse any exposed skin under tap water for 2 3 minutes. B. Make Carbonate-free Water
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This note was uploaded on 01/06/2011 for the course CHEM 223 taught by Professor Scheeline during the Fall '08 term at University of Illinois at Urbana–Champaign.

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Lab3_Acid_IDa - Chem 223 Identification of an Unknown...

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