lab 3 the galvimetric determination of calcium POST

lab 3 the galvimetric determination of calcium POST - The...

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The Gravimetric Determination of Calcium By: Jason Hsieh BB5 10/27/09 1
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Abstract : This lab’s purpose was using gravimetric analysis (analysis of masses of reactants and products to quantitatively determine the composition of a substance) to determine the percent of CaO and CaCO 3 of an unknown sample of an impure calcium compound. To do this, the experiment is designed to convert the calcium in the sample to CaC 2 O 4 . H 2 O. (1) Ca 2+ (aq) + C 2 O 4 2- (aq) + H 2 O (l) →CaC 2 O 4 . H 2 O (s) In order to maximize yield and purity, a technique was used in which urea was added to the solution. (2) (NH 2 ) 2 CO (s) + H 2 O (l) → CO 2(g) + 2NH 3 The ammonia product is able to bind with the hydrogen ion from HC 2 O 4 - (formed under acidic solutions) to slowly form the oxalate ion. The precipitate that results from these reactions is then filtered, dried and weighed. The results obtained were 51.17% with a standard deviation of ± 4.24% of CaO by mass and 91.32% of CaCO 3 by mass in the unknown sample with a standard deviation of ± 7.59%. Introduction: This lab entailed the use gravimetric analysis to determine the calcium composition of an impure sample of calcium carbonate (CaCO 3(s) ) by converting the calcium ion into calcium oxalate monohydrate (CaC 2 O 4 . H 2 O) and using homogeneous precipitation to crystallize the product. The reaction is as follows: Ca 2+ (aq) + C 2 O 4 2- (aq) + H 2 O (l) CaC 2 O 4 . H 2 O (s) (1) From here one can calculate the percent calcium in the original sample. Gravimetric analysis uses masses of reactants and products to quantitatively determine the composition of a substance. It has 3 requirements: (1) the compound must be converted into an isolatable/insoluble compound, (2) the precipitate must be pure, and (3) the precipitate must be in a form easily handled and weighed. To do this analysis, the reaction took place by heating it to 100 ° C. Also, in order to convert the calcium slowly to maximize yield and purity, urea (source of ammonia) was added to acidic conditions to generate the oxalate ion slowly. (NH 2 ) 2 CO (s) + H 2 O (l) → CO 2(g) + 2NH 3 (2) 2
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Adding ammonium oxalate with the calcium ion would not produce a product due to a low oxalate ion concentration in acidic conditions, (it was instead converted into HC 2 O 4 - and HC 2 O 4 ). As a solution, urea was added to the solution as a source of ammonia that can operate as a compound that raises pH slowly, and therefore slowly increases the oxalate ion.
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