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

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Introduction

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 Br�nsted-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. ...read more.

Middle

+ 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 the original acid = concentration of the diluted acid Materials : 50.0cm� of diprotic acid, 25.0cm� 0.10moldm-� NaOH, distilled water, 5mL phenolphthalein indicator Apparatus : 100cm� beaker, measuring cylinder, pipette, conical flask, burette, filter funnel, 250cm� volumetric flask Fair testing : A constant volume and concentration of NaOH is used throughout the experiment. Procedure Phase 1 1. 50.0cm� diprotic acid is measured by using measuring cylinder. 2. Then the acid is transferred into a 250cm� volumetric flask. 3. The volume of the solution is made up to the calibration mark of the volumetric flask with distilled water. The contents of the volumetric flask is swirled repeatedly. Phase 2 1. 25cm� of 0.10moldm-� of sodium hydroxide is pipetted into a conical flask. ...read more.

Conclusion

Hence the result is affected at the end of the experiment. Another reason is that the stated value 0.50M might not be the true concentration of the diprotic acid due to unavoidable error in the process of earlier dilution of the initial concentration of the acid. 5. During the process of titration, the acid added is a little bit in excess because the colour of phenolphthalein takes some time to undergo change. Swirling of the solution should be longer to observe the colour change, however the rate of acid drop in this experiment is continuous without pausing to observe the colour change. Precaution method : 1. The burette is clamped vertically to ensure accurate reading. 2. The conical flask is frequently swirled to ensure that all the acid added from the burette can react with the base and to ensure complete mixing of the reactants. 3. More than one accurate titration is carried out, to minimize error by getting accurate readings. 4. The unknown diprotic acid is handled carefully. Conclusion : The concentration of the diprotic acid can be determined through the acid-base titration. The concentration obtained in this experiment is 0.44M instead of the stated value 0.50M. ...read more.

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