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FInding the percentage purity of CACO3 in egg shell

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Aim Egg shell is hard and cracks easily when beaten because of the constituent of CaCO3.Our main aim will be to determine the purity or the percentage composition of CaCO3 in the egg shell by using the technique of back titration. The egg shell is just a representation of any substance containing CaCO3 in their constitution; this method could be used to find the percentage purity of marble, limestone, sea shell etc. Identification of Variables Independent variable 1. Purity of CaCO3 in the egg shell Dependent Variable 1. Volume of Hcl Controlled Variable 1. Concentration of Hcl The concentration of Hcl that is going to be used must be calculated for the experiment for show results. I took 4.931 g of egg shell which is 0.04931 moles. We will be following the procedure of back titration in this experiment since CaCO3is insoluble in water/reacts slowly with acid. Thus the first procedure of back titration is to react the substance with excess of acid , which means that we have to take a slightly high concentration of a preferred acid , in this experiment we have taken 2M of Hcl acid. ...read more.


16. Now pour the solution of egg shell and acid from the beaker in to the glass funnel carefully. 17. The solution is being filtered , it will take time. 18. The solution will not however add up to 250cm3 volume, so we need to add distilled water into the volumetric flask. Make sure that the water that is added stays under the filter paper level so as that it is being filtered. 19. After the solution has reached the 250cm3 , take a stirrer bar drop it in the volumetric flask , put the flask on the stirrer and let the solution stir for 2 hours. 20. Now take a pipette, make sure that the pipette is clean. 21. Rinse the pipette 22. Take a conical flask of 250cm3 and rinse it. 23. Using the pipette transfer the solution into the conical flask. 24. Take a burette and rinse it with NaOH , the desired alkali. 25. Put the plastic funnel on top of the beaker and carefully pour the alkali into the beaker. 26. Make sure that the meniscus is just above the zero mark. ...read more.


Back titration is a titration done in reverse; instead of titrating the original sample, a known excess of standard reagent is added to the solution, and the excess is titrated. A back titration is useful if the endpoint of the reverse titration is easier to identify than the endpoint of the normal titration, as with precipitation reactions. Back titrations are also useful if the reaction between the analyte and the titrate is very slow, or when the analyte is in a non-soluble solid. First measured mass of egg shell was taken and it was reacted with excess of HCl acid in a beaker. The solution was then taken and the volume was increased to 250cm3.Then using a pipette 25cm3 volume of the solution was taken and was poured into a volumetric flask and it was titrated against a known concentration of NaOH.This was repeated 7 times and the mean titre was found to be 21.11cm3. After finding the titre calculations were done to find the mass of CaCO3 present in the egg shell. The egg shell weighed 4.931 g ± 0.001 and was found to contain 4.750 g ± 0.1653 g CaCO3 . The % purity of the egg shell containing CaCO3 was found by using the give formula :- (Mass of CaCO3/Mass of egg shell) x 100 which gave 96.32% purity. ...read more.

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