Determination Of Na2Co3 In Soda Ash: Back Titration T - Laboratory Report PDF

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Rachel Tammone 10/6/14 CHM114 FX Heather Pedziwiatr Determination of Na2CO3 in Soda Ash: Back Titration Technique Purpose: Soda ash, sodium carbonate or Na2CO3, is an impure and important commercial inorganic chemical. It is a raw material of chemicals, glass, detergents, and other important industrial products. Soda ash’s impurities can be titrated with a strong acid, but the titration occurs in two stages. The easiest way to perform these two stages is to use a back titration. Using this kind of titration requires excess acid to be added and allowed to react completely with the carbonate. The solution is briefly boiled to remove the CO2. After cooling, the excess acid still present in the solution is titrated with NaOH. The purpose of this lab it to perform a successful back titration and then use the data collected from the titration to calculate the perfect Na2CO3 in the unknown sample. Theory: A back titration has to be performed to titrate CO3- with a strong acid. The titration comes in two stages: CO32- + H+  HCO3- + H+  H2CO3  H2O + CO2 The following formulas are also used to analyze the data: Volume HCl added X (Mol HCl/ml) = mmol HCl added Volume NaOH added X (Mol NaOH/ml) = mmol NaOH added Net mmol HCl to titrate soda ash = volume NaOH added – mmol NaOH added Mmol Na2CO3 titrated = net mmol HCl to titrated soda ash X (1 mmol Co32-/2 mmol HCl) % Na2CO3 = (g Na2CO3 in sample/ .4016) X100 Procedure: 1. Dry the unknown sample of soda ash for 1.5 hours to constant weight in the drying oven at 120C & allow to cool before weighing 2. Use a tarred weigh boat to accurately weigh two about .4g samples of the unknown to four decimal places & fully transfer each sample into a clean & dry 250 mL Erlenmeyer flask that has been labeled 3. When ready to titrate, add about 50 mL of distilled water, dissolve by swirling the solution, & add 3 drops phenolphthalein 4. Rinse & fill a buret with your standardized HCl solution & titrate the first soda ash sample to the disappearance of the pink color 5. Add an additional volume of HCl about equal to the amount added in Step 4, then add 10 mL more HCl 6. Bring solution to a boil & boil 5 minutes. Cool to room temperature & add 3 drops phenolphthalein 7. Rinse & fill a buret with your standardized NaOH solution. Back titrate the cooled soda ash sample to the phenolphthalein endpoint (pink color) 8. Calculate & report the % Na2CO3 in the sample Data/Calculations: Table 1 shows the data collected from performing the back titrations. The titrations were performed 3 times to try to obtain accurate data.

Trial Number Sample Weight Final Buret reading HCl Initial Buret reading HCl Volume HCl added mmol HCl added Final Buret reading NaOH Initial Buret reading NaOH Volume NaOH added mmol NaOH added Net mmol HCl to titrate soda ash Mmol Na2CO3 titrated Calc g Na2CO3 in sample Calc % Na2CO3 in sample

1 .4468 g

2 .4292 g

3 .3868 g

36.98 mL

35.89 mL

38.21 mL

1.21 mL

1.29 mL

4.18 mL

35.77 mL 4.3389 mmol

34.60 mL 4.1970 mmol

34.03 mL 4.1278 mmol

9.99 mL

19.18 mL

31.29 mL

1.40 mL

9.99 mL

19.18 mL

8.59 mL

9.19 mL

12.11 mL

.8693 mmol

.9300 mmol

1.2255 mmol

3.4696 mmol

3.2670 mmol

2.9023 mmol

1.7348 mmol

1.6335 mmol

1.4512 mmol

.1837 g

.1730 g

.1537 g

45. 74 %

43. 0776 %

38.27 %

The data was obtained by using the following calculations. The sample calculations use trial 1 data. All data was analyzed in the same manor.  Volume HCl added X (Mol HCl/ml) = mmol HCl added o 35.77 X (.1213 M/ mL)  4.3389 mmol HCl added  Volume NaOH added X (Mol NaOH/ml) = mmol NaOH added o 8.59 X (.1012 M/ mL)  .8693 mmol NaOH added  Net mmol HCl to titrate soda ash = volume NaOH added – mmol NaOH added o 4.3389 - .8693  3.4696  Mmol Na2CO3 titrated = net mmol HCl to titrated soda ash X (1 mmol Co32-/2 mmol HCl) o 3.4696 X (1 mmol Co32-/ 2 mmol HCl)  1.7348 mmol Na2CO3 titrated  G Na2CO3 in sample = mmol Na2CO3 titrated X (molecular mass Na2CO3/mL) o 1.7348 X (105.9 / mL)  183.7 mg  .1837 g  % Na2CO3 = (g Na2CO3 in sample/ .4016) X100 o (.1837/ .4016) X 100

 45.74 % The average Calc % Na2CO3 in sample was found using only the first two trials data  (45.74 + 43.0776) / 2 o 44.4088 % Na2CO3 in unknown sample Discussion/Error Analysis: Possible sources of error could have come from the balances used to weight the sodium carbonate. If the balances were not calibrated correctly, the weight of the sodium carbonate would be incorrect and this would result in all further calculations being wrong. Another possible source of error could have been using in properly standardized solutions of HCl and NaOH. If the burets contained gas bubbles, the volume of NaOH and HCl would be incorrect. This would skew all calculations done afterwards. Conclusion: Back titrations can be useful when titrating samples that have two stages and impurities that need to be titrated out. Overall, this experiment was successful in teaching how to calculate percent sodium carbonate in an unknown sample by performing a back titration....