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Determination of the Amounts of Sodium Hydroxide Solution and Sodium Carbonate in a Mixed Solution

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Introduction

Experiment 2 Determination of the Amounts of Sodium Hydroxide Solution and Sodium Carbonate in a Mixed Solution Date of Experiment: 21/9/2005 Objective: To determine the percentage by mass of Sodium Hydroxide and Sodium Carbonate in a mixed solution. Introduction Sodium carbonate and sodium hydroxide are both alkaline in nature. When hydrochloric acid is added to the mixture, sodium hydroxide reacts like this: NaOH + HCl -----> NaCl + H2O ....................... (1) Sodium carbonate reacts with hydrochloric acid in a similar way but the reaction comes in 2 steps: Na2CO3 + HCl ----------->NaHCO3 + NaCl............. (2) NaHCO3 + HCl ----------->NaCl + H2O + CO2.......... (3) The pH conditions in which reactions (1) and (2) take place are approximately the same: they both occur at a relatively higher pH (at a pH value higher than 7). Reaction (3) is quite different from the other 2 reactions. It takes place at a lower pH (at a pH value lower than 7). So, using the 2 acid-base indicators, the end points for the reactions can be found. Phenolphthalein turns from pink to colorless as the pH value changes from 10 to 8. Methyl orange turns from yellow to orange when the pH value drops from about 4 to 3. ...read more.

Middle

So, there are also 5.033 X 10-4 moles of Na2CO3 in the experiment. Mass of Na2CO3 in the experiment: 5.033 X 10-4 X (23 X 2 + 12 + 16 X 3)= 0.5335g Mass of NaOH in the experiment: (1.3651 X10-3 - 5.033 X 10-4) X (23 + 17)= 0.03442g So, the percentages by mass of NaOH and Na2CO3 are 39.15% and 60.85% respectively. Method 2 A table showing the volume of HCl used for reacting both Na2CO3 and NaOH 1 2 3 Final reading 22.6 cm3 40.8 cm3 19.7 cm3 Initial reading 3.1 cm3 21.6 cm3 0.5 cm3 Volume of HCl used 19.5 cm3 19.2 cm3 19.2 cm3 (Due to time limitations, only 3 titrations could be done and the first titration is not rejected) Average volumes of HCl used = (19.5 + 19.2 X 2)/ 3 = 19.3 cm3 No of moles of HCl used = 19.3/1000 X 0.09868 = 1.905 X 10-3 moles A table showing the volume of HCl used for reacting NaOH. 1 2 3 Final reading 27.7 cm3 35.5 cm3 43.6 cm3 Initial reading 19.7 cm3 27.7 cm3 35.5 cm3 Volume of HCl used 8 cm3 7.8 cm3 8.1 cm3 (Due to time limitations, only 3 titrations could be done and the first titration is not rejected) ...read more.

Conclusion

Initially, the solution in the flask was alkaline. So, the indicator was pink at the start. After the end point was reached by adding HCl into the flask, the solution had a lower pH and the indicator turned to colorless. 2. Explain why the titration should be carried out slowly and with constant stirring in (b). This can make sure that all sodium hydroxide have reacted before further adding hydrochloric acid into the conical flask. After adding barium chloride was added into the conical flask, precipitates were formed. These precipitates would hinder the reaction of sodium hydroxide and hydrochloric acid. So, in order to allow all the hydrochloric acid to be reacted before anymore of it is added, we need to stir it and do it slowly. 3. Suggest with reasons which method is more accurate in the experiment. I think that method 2 is more accurate. This is because method 1 involves 2 end points in 1 titration. If an error was made in the first part of the titration in method 1, the results obtained in the second part would also be wrong. Also, CO32- reacts in 2 steps. But how will I be able to confirm that some HCO32- has not further reacted to form carbon dioxide and water in the first part? This also lowers the accuracy of method 1. ...read more.

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