HShin_Hoon_Shin_Lab_3_Report - Author Hoon Shin TA Eileen...

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Author: Hoon Shin TA: Eileen Chen Section: A05 Class: Chem100A, UCSD(University of California San Diego) Date: 4/28/08 Mixture of Carbonate/Hydroxide and Citric Acid in Soft Drinks Abstract Main purpose of this experiment is for one to calculate the concentration of CO 3 2- and OH - of unknown sample # 59 by performing two different titrations with two different indicators: phenolphthalein and bromocresol green. The titration was carried with HCl, standardized with Na2CO3, which had concentration of 0.09010M ± 0.03674M. As a result, The concentration of CO 3 2- and OH - in unknown sample #59 turned out to be 0.228M ± 1.34E-05 and 1.01M ± 1.34E- 05 respectively. For the second part of this experiment was to find the grams of citric acid in 7-up soda can by titrating with NaOH which has concentration of 0.0099M ± 0.007275M. The average citric acid per can has determined to have 0.3798g Introduction When the Carbon dioxide reacts with CO 2 , it forms carbonic acid, H 2 CO 3 . Equations are following: CO 2 (aq) + H 2 O   H 2 CO 3 (aq) H 2 CO 3 (aq) + H 2 O   HCO 3 - + H 3 O + HCO 3 - + H 2 O   CO 3 2- + H 3 O + In this formula, HCO 3 - is conjugate base. If this conjugate base is mixed with NaOH solution, carbonic acid will be deprotonated and will produce or remain as carbonate ion. To find the concentration of CO 3 2- in a solution of sodium hydroxide, one must find out the amount of base present in the solution. To find both the concentration and the base, one must use two indicators. If one uses only the phenolphthalein, a basic indicator, when titrating with
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HCl, the CO 3 2- will be changed only to bicarbonate, and hydroxide will be change to only H 2 O. The equations for this reaction are following: CO 3 2- + H +   HCO 3 - OH - + H +   H 2 O Setting “X” equals to the moles of carbonate, and “Y” equals sodium hydroxide of the unknown, sum of X and Y would give moles of standardized acid. This is instance when the basic indicator such as phenolphthalein is used. However, if the acidic indicator such as bromocresol green is used, CO 3 2- will change to H 2 CO 3 . Thus, 2X+Y would give the moles of standardized acid because two hydrogen ions are needed to form carbonic acid. During the titration, when CO 3 2- becomes H 2 CO 3 with bromocresol green indicator, the color will change from blue to green. This might be the end point, but it does not mean that all the solutions has used up. However, some CO 3 2- will change to HCO 3 - . To titrate this left over of carbonate, one must remove all the bicarbonate in the solution by using this equation: H 2 CO 3 (aq)   H 2 O + CO 2 (aq) With the equation above, it is obvious that one can eliminate carbonic acid by boiling the solution. As a result, endpoint will be better or clear for one to observe. Experimental
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HShin_Hoon_Shin_Lab_3_Report - Author Hoon Shin TA Eileen...

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