Back titration - chem lab report PDF

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REGINALD AIKINS SECTION: JX TA: YETING ZHENG OCTOBER 7, 2019

Preparing Standardized Solutions Purpose and theory: Soda ash, also known as 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 CO 2. 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. 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 = (mass of Na2CO3 in soda ash sample/original mass of soda ash) 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. Repeat steps 3-7 for second sample of dilute impure soda ash solution. 9. Calculate & report the % Na2CO3 in the sample

Data and Calculations: Attached

Discussions and Conclusions: The % values gotten from both titrated Soda ash sample were very close and consistent which showed that the back titration was conducted well. There could have been some possible errors though. Errors 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. 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....