5 Determination OF THE Percentage OF Purity OF A Sample OF Calcium Carbonate PDF

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DETERMINATION OF THE PERCENTAGE OF PURITY OF A SAMPLE OF CALCIUM CARBONATE

NAMES: KABO ISHMAEL LUPE STUDENT ID: 18000327 COURSE: PURE AND APPLIED CHEMISTRY

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AIMS 

To determine the percentage purity of a sample containing calcium carbonate using a volumetric analysis called reverse titration.

INTRODUCTION According to Asaki and Hioki, (2013), back titration is method of volumetric analysis used to determine the concentration of an analyte by reacting it with a known number of moles of excess reagent. The excess reagent is then titrated with the second reagent. The concentration of the analyte in the original solution is then related to the amount of reagent consumed. Also back titration is 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. Back titration is useful if the endpoint of the reverse is easier to identify than the endpoint of the normal titration as with precipitation reactions and also useful if the reaction between the analyte and the titrant is very slow or when the analyte is a non-soluble solid. (Needham, 2013). Calcium reacts with a very strong mineral acid such hydrochloric acid or sulphuric acid to form carbon dioxide and water CaCO3 + 2HCL

CaCl2 + CO2 + H2O

However, the reaction requires heat to ensure it reaches completion and hence the amount of HCl and carbon dioxide escapes into the atmosphere. Then resulting solution is titrated with sodium hydroxide to determine how much hydrochloric acid remains HCl

+

NaOH

NaCl + H2O

The amount of HCl reacting with the carbonate is calculated from the difference between the amount of HCl added to Calcium Carbonate and the amount of acid remaining when the reaction is complete. In this way the amount of Calcium Carbonate present can be determined. APPARATUS AND REAGENTS        

CaCO3 25 mL Pipette 250mL volumetric flask Funnel Burette 3 x 250mL conical flask Analytical balance NaOH

PROCEDURE A mass of 0. 9946g of impure CaCO3 sample was weighed accurately using an analytical balance and added into a conical flask. Measured 20mL of distilled water using measuring cylinder was added to the flask together with 50mL pipetted volume of HCl. The flask was swirled gently until no effervescence was observed and the solid had disappeared. The 2|Page

mixture was transferred into 250mL volumetric flask, then the conical flask was rinsed with distilled water and was added to the volumetric flask. Distilled water was used to fill the volumetric flask to the mark. The solution was mixed thoroughly. Thereafter 20mL of the mixture was pipetted in the conical flask and 3 drops of phenolphthalein were added to each flask. The mixture was titrated with sodium hydroxide until it turned slightly pink. DISCUSSION OF RESULTS This experiment is deemed useful in the determination of analyte indirectly (back titration). A mass of 0.9946g of the sample was used to determine the percentage purity of the sample. According to the presentation of results in the sample calculation sheet, firstly, number of moles of the HCl were before adding to the sample by taking the product of the given volume and concentration. That is the moles of HCl unreacted (0.0405mol). The titration between HCl and NaOH was useful in calculating the amount of excess moles left when HCl reacted with CaCO3, which were computed to be 0.00245mol. A ratio of volumes were used to determine moles of 20mL to 250ml and was 0.025mol. Theoretical mass of CaCO 3 is 1.0260g. Comparing the actual and theoretical mass, there is no doubt that about my result. Percentage purity of CaCO3 is 96.9% which means there some impurities in the sample of CaCO3. Possible sources of errors were identified and accounted for, during the experiment. During the titration between HCl and NaOH, parallax error occurred and it was minimised by taking the volume average of the 3 best titration measurements. Corwin, (2009). One of the most common error committed was overshooting because the colour of the solution was not visible. As a way of accounting for this error, 3 drops of the indicator were added to increase visibility when there is a slight colour change. (Akitt, 2006). CONCLUSION The obtained results are acceptable and has met the objective of the experiment. The average volume used in titration is 0.2226mL. Percentage purity the sample is 96.9% and this shows that there are less impurity in the sample containing CaCO3. Back titration is a unique technique that is applicable in volumetric analysis of samples which are non-soluble solids. REFERENCES AKITT, J.W. (2006). ChemInform Abstract: TEMPERATURE DEPENDENCE OF PROTON CHEMICAL SHIFTS OF AQUEOUS SODIUM FLUORIDE, SODIUM HYDROXIDE AND TETRAMETHYLAMMONIUM HYDROXIDE, THE INTERNAL TETRAMETHYLAMMONIUM ION STANDARD. Chemischer Informationsdienst, 4(39), p.no-no. Asakai, T. and Hioki, A. (2013). Assay of high-purity sodium oxalate traceable to the international system of units by coulometric titration and gravimetric titration. Microchemical Journal, 108, pp.24–31 Corwin, C.H. (2009). Introductory chemistry : concepts & connections : laboratory manual. Upper Saddle River, N.J.: Pearson Prentice Hall. Needham, J. (2013). Grand titration : science and society in east and west. Place of Publication Not Identified: Routledge. 3|Page

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