Experiment 3-1

Experiment 3-1 - Experiment 3 Mixture of Carbonate and...

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Experiment 3: Mixture of Carbonate and Hydroxide Abstract This experiment involved determining the CO 3 2- concentration in a sodium hydroxide solution by also determining the OH - concentration through two sets of titrations utilizing different indicators, bromocresol green and phenolphthalein. The titrations were done using a standardized solution of HCl using bromocresol green as the indicator. In standardization, the HCl was titrated against the primary agent Na 2 CO 3 and was determined to have a concentration of 0.09642 M ± 0.00465 M . Unknown #43 was thus determined to have a [OH - ] of 0.8864 M and a [CO 3 2- ] of 0.0119 M . The low value calculated for [CO 3 2- ] is an indication that detecting the endpoint of the solutions was difficult thus incorrect volumes of titrant were delivered. Introduction When water is exposed to CO 2 , the following reactions occur which forms carbonic acid, H 2 CO 3 : 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 + The species bicarbonate, HCO 3 - , can act as either a weak acid or a weak base depending on the conditions of the solution. If it’s in a strong basic solution like sodium hydroxide, NaOH, it will exist as carbonate ions: CO 2 + 2OH -   CO 3 2- + H 2 O A solution of unknown concentration of CO 3 2- in sodium hydroxide is given and the task is to determine the concentration of CO 3 2- . But to do this, the amount of base must be determined. That means that there are two unknowns to be determined. In determining these two concentrations through titration, two different indicators must be used. The reason for this can be seen if only one indicator was used. So for example, in this experiment phenolphthalein, a basic indicator, will be used. With phenolphthalein, when the solution is titrated with hydrochloric acid (HCl), the carbonate ion will only protonate to become bicarbonate, HCO 3 - and the hydroxide will protonate to become water: CO 3 2- + H +   HCO 3 - OH - + H +   H 2 O If x equals the moles of CO 3 2- and y equals the moles of OH - , since hydrochloric acid is monoprotic this means that the number of moles of HCl needed to titrate both the carbonate ions and the hydroxide ions equals to: x + y = (volume HCl)[HCl] Looking at the equation, there is no way to solve for either x or y without being given more information. Thus a second equation is needed in order to solve for the two unknowns. This second equation will thus be obtained by utilizing a second indicator which is bromocresol green, which is known to be an acidic indicator. With bromocresol green, when the solution is titrated with hydrochloric acid (HCl), the carbonate ion will protonate to become carbonic acid, H 2 CO 3 , and the hydroxide will protonate to become water: CO 3 2- + H +   HCO 3 - OH - + H +   H 2 O In this instance, the number of moles of HCl needed to titrate both species is equal to:
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2 x + y = (volume HCl)[HCl] since it requires twice as much HCl to titrate the carbonate ions using bromocresol green:
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This note was uploaded on 04/15/2008 for the course CHEM 100A taught by Professor Dai during the Fall '06 term at UCSD.

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Experiment 3-1 - Experiment 3 Mixture of Carbonate and...

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