Formatting page ...
Back Titration A back titration is a titration method in which you determine the concentration of an unknown by reacting it with a known amount of excess reagent. You then titrate the remaining excess reagent with a second reagent. The second titration s results show how much of the excess reagent the unknown consumed in the first titration, and you can then calculate the unknown s concentration. You use a back titration when you need to determine the concentration of an unknown, and you have a known concentration of excess reactant. You typically use it in acid-base titrations when the unknown is volatile (e.g., ammonia), an insoluble salt (e.g., calcium carbonate), when it would be difficult to discern the endpoint of a direct titration (e.g., in a precipitation reaction), or when the reaction occurs very slowly. Example Problem You are trying to determine the concentration of ammonia in a commercially available cloudy ammonia solution. First, you pipet 25.00 mL of the cloudy ammonia solution into a 250 mL conical flask. You immediately add 50.00 mL of 0.100 mol/L HCl to the ammonia solution. Then you titrate the excess (unreacted) HCl with 0.0500 mol/L Na2CO3. The titration requires 21.50 mL of Na2CO3. Calculate the concentration of the ammonia in the cloudy solution. (Ans. = 0.114 mol/L) 1. Write the chemical reaction for the titration reaction 2. Calculate the moles, n, of Na2CO3 that reacted in the titration (n = cV) 3. Use the molar ratio of HCl:Na2CO3 from the balanced equation to determine the moles of excess HCl. 4. Calculate the total moles of HCl that you initially added to the cloudy ammonia solution 5. Write the balanced chemical equation for the reaction between ammonia and HCl 6. Use the molar ratio of NH3:HCl to calculate the moles of NH3 that reacted with the HCl (nreacted = ntotal ntitrated) 7. Calculate the ammonia concentration in the cloudy solution.
|
