Objective : To determine the concentration of an unknown diprotic acid using acid-base titration.

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Objective         : To determine the concentration of an unknown diprotic acid using acid-base titration.

Introduction   :  Volumetric analysis is a quantitative method of chemical analysis. A solution  of accurately known concentration is reacted with a solution of the substance whose concentration desired. By comparing the reacting volumes, the concentration of the unknown can be determined. The actual procedure of adding one solution to the other for a complete reaction is called a titration. The two types of titrations are acid-base titrations and redox titrations.

          In this experiment, the molar mass of a diprotic acid is measured by acid-base titration. This method exploits the BrØnsted-Lowry acid-base theory. In this model, any acid is defined as a proton(hydrogen ion, H⁺) donor, and a base is defined as a proton acceptor. When an acid and base react, protons are transferred from the acid to the base.

                                                           HA + B⁻→ A⁻ + HB

          The resulting solution is neutral, it has neither acid nor basic properties. The point at which the reaction is complete is referred to as the equivalence point, reactants are in stoichiometric proportions, this is often not visible, as acids and bases are colourless and so an indicator is required. Since the products of an acid-base reaction may be acidic, basic or neutral, it is important to choose an indicator so that the end point matches the equivalence point. An appropriately chosen indicator will show a colour change(end point) when the reaction is complete(equivalence point). For example, phenolphthalein, an acid-base indicator turns pink in neutral or slightly basic solutions. When a very slight pink colour is seen in the presence of phenolphthalein, equimolar amounts of acidic protons and base are present. If the molar concentration(in moles/litre) and the volume of base needed to neutralized is known, the number of moles of acid neutralized can be calculated. Hence, the concentration of the diprotic acid can be calculated.

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  In this experiment, a standard solution of a diprotic acid is prepared,   and then titrated with a 25.0cm³ sample of 0.10moldm⁻³ sodium hydroxide solution. The diprotic acid reacts with sodium hydroxide as follows:

H₂A (aq) + 2NaOH(aq)→ Na₂A(aq) + 2H₂O(l)

          From the equation, the number of moles of diprotic acid is half the number of moles of NaOH, then from titration, the volume of the acid needed to neutralize the NaOH is used to determine the molarity of the diluted diprotic acid.

        Concentration of the               diluted acid  


Concentration of ...

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