why do we use back titration instead of direct titration

In a direct titration, you add a standard titrant to the analyte until you reach the end point. In a back titration, you add an excess of standard titrant to the analyte, and then you titrate the excess titrant to determine how much is in excess. one of the reactants is volatile and some might escape during the titration (e. g. , NHв)
the analyte is an insoluble salt (e. g. , CaCOв) Here's an example of a back-titration to determine the mass of calcium carbonate present in a sample of chalk. She placed the sample in a 250 mL conical flask and added 50. 00 mL 0. 2000 mol/L HCl from a volumetric pipette. She then titrated the excess HCl with 0. 2500 mol/L NaOH and found an average titre of 40. 00 mL. She added 0. 010 00 mol of HCl to the chalk. She used 0. 008 000 mol of NaOH to neutralize the excess HCl. The chalk must have reacted with 0. 002 00 mol of the HCl.

Since 1 mol of CaCOв reacts with 2 mol of HCl, there must have been 0. 001 00 mol of CaCOв. The mass of CaCOв in the chalk was (0. 001 00 Г 100. 09) g = 0. 100 g. Back titration is a technique which can be used in the analysis of solids that are insoluble, or only partly soluble in water. If the solid can react with an acid it may do so fairly slowly, so that ordinary methods of titration cannot conveniently be used. Under these circumstances, the solid can be made to react with an excess of acid. Since an excess is used, the reaction may be fairly rapid. When the reaction is complete, the remaining acid may be determined by titration with alkali. From the volume and concentration of alkali used, the composition or purity of the original solid can be found. In this example, a mixture of calcium oxide and silicon dioxide is analysed to determine the percentage by mass of calcium oxide present in the mixture.

The method can be applied because calcium oxide reacts with acids but silicon dioxide does not. Open Yenka file. The display shows a small beaker that contains a mixture of calcium oxide and silicon dioxide. There is a balance which is set to ignore the mass of any container - it shows the mass of contents only. There is a bottle of acid which will react with the calcium oxide, and titration apparatus to determine the quantity of unreacted acid. Place the beaker on the balance. What is the mass of solid in the beaker, in grams? Now return the beaker to its original position and add the acid to it. How can you tell from the pH that a reaction is taking place in the beaker? Write a balanced equation for the reaction that has taken place.

Now add the contents of beaker 1 to beaker 2. Identify the brown sediment that falls rapidly to the bottom of beaker 2. Now open the burette, which contains 0. 1 mol / litre sodium hydroxide solution, using the control at the right of the burette. Allow a total of about 25 cm of alkali to enter the beaker. What volume of sodium hydroxide is required to reach the endpoint (the near vertical portion of the graph)? a) Write a balanced equation for the neutralisation reaction. b) Calculate the number of moles of alkali added. c) How many moles of HCl were therefore present? d) How many moles of HCl were initially present, before it was added to the solid? e) How many moles of acid reacted with the solid? a) Use the equation in Q3 and your answer to the last question to calculate the number of moles of calcium oxide which must have been present. b) Calculate the mass of 1 mole of calcium oxide. (Relative atomic masses are available from the Periodic Table, which can be viewed by clicking the relevant icon in the main toolbar. ) c) Calculate the mass of calcium oxide initially present.

Calculate the percentage by mass of calcium oxide in the original mixture. Back titration is used in the analysis of compounds that are insoluble in water but soluble in acid. (There are other applications as well. ) An excess of acid is added to the solid and the remaining acid is titrated against a standard solution of alkali. This allows the acid remaining, and hence the acid used, to be calculated. Knowledge of the stoichiometry of the reaction between the solid and the acid allows determination of the number of moles of solid present.

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