Back Titration -...

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Back Titration (Indirect Titration) Calculations Key Concepts A back titration, or indirect titration, is generally a two-stage analytical technique: a. Reactant A of unknown concentration is reacted with excess reactant B of known concentration. b. titration  is then performed to determine the amount of reactant B  in excess . Back titrations are used when: one of the reactants is volatile, for example ammonia. an acid or a base is an insoluble salt, for example calcium carbonate a particular reaction is too slow direct titration would involve a  weak acid - weak base titration       (the end-point of this type of direct titration is very difficult to observe) Example : Back (Indirect) Titration to Determine the Concentration of a Volatile Substance A student was asked to determine the concentration of ammonia, a volatile substance, in a commercially available cloudy ammonia  solution used for cleaning.  First the student pipetted 25.00mL of the cloudy ammonia solution into a 250.0mL conical flask.  50.00mL of 0.100M HCl (aq)  was immediately added to the conical flask which reacted with the ammonia in solution.  The excess (unreacted) HCl was then titrated with 0.050M Na 2 CO 3(aq) 21.50mL of Na 2 CO 3(aq)  was required.  Calculate the concentration of the ammonia in the cloudy ammonia solution. Step 1: Determine the amount of HCl in excess from the titration results a. Write the equation for the titration:  2HCl (aq) + Na 2 CO 3(aq) 2NaCl (aq) + CO 2(g) + H 2 O (l) acid + carbonate salt + carbon dioxide + water b. Calculate the moles, n, of Na 2 CO 3(aq)  that reacted in the titration: 
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This note was uploaded on 02/02/2012 for the course CHEM 211 taught by Professor Santos,j during the Spring '11 term at Abu Dhabi University.

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Back Titration -...

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