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Estimating the % purity of marble by back titration method

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Chemistry Lab - Conclusion and Evaluation Estimating the % purity of marble by back titration method Trial No. Percentage Purity of CaCO3 Uncertainty 1 90.37% �4.81% 2 95.11% �4.86% Average 92.74% �4.84% The values for the % purity of marble that I have calculated lie in a close range, however there is a significant difference, of 4.74% in the purity estimated between the 2 individual values. Nonetheless, the deviation is covered by the uncertainty of �4.84%. However, certain errors have occurred that have caused this deviation in the final results. REASONS FOR DEVIATION Random Errors 1. Loss of marble after weighing: After weighing out the marble, there is a small loss in mass in transferring the mass from the butter paper used for weighing as ...read more.


3. Air bubbles in pipette dispenser: For this experiment, due to the corrosive nature of the concentrated acid, a pipette dispenser was used to measure out the HCl. However there were some air bubbles that were trapped that cause an error in the volume of HCl that is not accounted for by the absolute uncertainty of the pipette. Systematic Errors 1. Unevenly crushed marble: The CaCO3 provided was not evenly crushed; some was still in larger chunks while the rest was finer granules. This non-uniformity in the texture of CaCO3 also creates a different surface area for the acid to react with and this can be the cause of the differing values of purity. ...read more.


2. Carefully using apparatus to avoid human errors: By practice and careful usage one can eliminate the errors caused by the air bubbles in the pipette, errors in transfer and parallax. 3. Marble should be crushed evenly: the CaCO3 should all be of the same texture so that the surface area is kept controlled and this will reduce the disparity in the purity values that have been caused by the differing surface areas. 4. Using another indicator that prevents uncertainty in the exact point of neutralization: If a solution like Universal Indicator is used then the exact point when the titration is complete can be easily identified and thus by eradicating this error, one can aim to reduce the disparity observed in the readings. ...read more.

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