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# Estimating the % purity of marble by back titration method Essay

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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 ï¿½4.84%. 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 a small quantity of the fine powdered marble sticks to the butter paper and does not react with the HCl at all.

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Also, while transferring the crushed CaCO3 some particles could have also been blown away by the wind. Thus the disparity in the masses used could account for the different values obtained.

2. HCl left in the pipette: being a manual instrument, there is no definite way of making sure that each and every drop of the acid that was measured has been transferred to the conical flask, this reduces the volume of acid used and such a change in any one of the reading will directly affect the value of the % purity.

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.

2. Excess NaOH added in titration: While titrating the HCl and the HCl + CaCO3 solutions the usage of the phenolphthalein indicator causes an error in the amount of alkali added as the titration is only stopped once the colour changes to a pale pink, this firstly is a very qualitative and subjective instruction and also the indicator on turning pink indicate a slight alkalinity. Hence, the actual neutralization point was left behind 1-2 drops back.

3. Unevenly distributed impurities: CaCO3 is found naturally as marble in the earth. Thus, being a natural stone the impurity that it will contain will be randomly distributed and due to this unevenness, the purity will vary with every sample.

IMPROVEMENTS TO PREVENT AFOREMENTIONED ERRORS

1. Keep fans and windows shut and cover CaCO3 while transporting: By doing this one can minimize the loss of particles and thus reducing the uncertainty.

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.

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