Determination of Na2CO3 (Full Report)

Chance Boudreaux

DETERMINATION OF Na₂CO₃ BY MEANS OF BACK TITRATION Chance Boudreaux, Kennesaw State University Quantitative Analytical Chemistry Lab 2800 Instructor: Dr. H. Z. Msimanga INTRODUCTION: The objective of the experiment is to determine the % mass of sodium carbonate (Na₂CO₃) in the unknown sample. This is done by means of a back titration in order to ensure that the titration will be successful due to the possibility that the carbonate ion (CO₃²¯) can form two different reactions with hydronium (H₃O⁺). Back titration is a titration done in reverse. Instead of titrating the original sample, a known excess of standard reagent, in this case HCl, is added to the solution, and the excess is titrated. Using a back titration is helpful if the endpoint of the reverse titration is easier to identify than the endpoint of the normal titration. Back titrations are also helpful if the reaction between the analyte and the titrant is very slow, or when the analyte is in a non-soluble solid. Either of these two reactions can occur:

(1)

CO3

2

(2)

CO3

2





  HCO  H 3O      HCO3



  HCO 2 H 3O      HCO3





 H 2O



 H 2O

These reactions cause a problem due to eq1 needing the indicator phenolphthalein and eq2 using indicator methyl orange.

1

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Chance Boudreaux

In order to perform a back titration of sodium carbonate, a standardized strong acid and base will  be required. The data, in this report, was extracted ex tracted by the use of 0.09731M HCl and 0.09948M 0.0994 8M  NaOH (this means phenolphthalein will be the indicator). The experiment will also require a 50mL buret, 10mL pipet, a graduated cylinder, 250mL volumetric flask, a balance, and a hot  plate for each time you run it. Table 1: (preparation of each flask for titration)

Table 1

Trial 1

Trial 2

Trial 3

Trial 4

(g)

0.3395

0.3579

0.3049

0.3049

HCl (mL)

50.00

50.00

50.00

50.00

H₂O (mL)

25.00

25.00

25.00

25.00

Na₂CO₃

After the solutions are prepared they should be heated for 3-4 minutes to allow the reaction (eq3) to go until completion, as well as, to allow the CO₂ (g) that is formed to dissipate. This is so that the CO₂ does not adversely react with the phenolphthalein creating an unwanted precipitate.

(3)

  2 NaCl  CO2   H 2O  Na2CO3  2 HCl   HCl   

After the solutions cool to room temperature begin titrating. The following equations (eq4, eq5, & eq6) are used to obtain grams of Na₂CO₃ from the sample after titrations are complete.

(4)

 M   NaOH   L  NaOH  1 mol  HCl  

1 mol  NaOH   M   HCl 

 L  HCl excess

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Chance Boudreaux

(6)

 M   HCl  L  HCl reacted  1 mol  Na2CO3  MW  Na2CO3



2 mol  HCl 

 g   Na2CO3

(MW Na₂CO₃= 105.9885 g/mol)

(7)

(8)

 g   Na2CO3  g  Total  Na2CO3 S  x  _ 

*100 %  mass % CO3

*100%  % RSD  RSD

 x From here find g Na₂CO₃, than convert to mass % Na₂CO₃ as shown in table 2. Table 2: (Experimental results of Trials 1-4, % mean and %RSD)

Table 2

Trial 1

Trial 2

Trial 3

Trial 4

 NaOH (mL)

24.52

24.42

24.50

24.38

 Na₂CO₃ (g)

0.1308

0.1340

0.1148

0.1146

mass Na₂CO₃ (%)

38.53

37.44

37.65

37.59

mean Na₂CO₃ (%)

37.80

%RSD (%)

±1

(Unknown code #249 for stock sample) DISCUSSION:

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Chance Boudreaux

sample of Na₂CO₃. The excess HCl was titrated with standardized NaOH, to quantify the amount of HCl that reacted with the sample. Using stoichiometry, the g of analyte (CO₃) can now be found and then converted to a % mass by dividing the total mass of the sample. The % CO₃ along with the %RSD, approximates the amount of CO₃ in a given sample from the stock bottle. The % relative standard deviation (%RSD) shows the tolerance (±) of a sample's mean. Due to the low %RSD of this data, it proves the data's mean to be a valid approximation. The %RSD of  this data also shows that most of the deviation is from the imperfect blend of the unknown sample and the tolerance error of the instruments used, due to a relatively small amount of  deviation in the mean. The other calculations supporting the precision of the data are done by a ttest. A t-test uses a normal distribution curve to see if your data fits within set parameters. The data acquired through this experiment test valid for the 50th confidence level, meaning that the data fits within the ±25% of the mean on the normal distribution curve. CONCLUSION: The results of the back titration concluded that we have a mean of 37.80% Na₂CO₃ with a ±1%RSD for the unknown Na₂CO₃ stock provided in the laboratory. The tcalc = 0.7407 for the 50% confidence level and the tcrit = 0.741 meaning that the data is appropriate for the 50th confidence level. REFERENCE: Harris, D. C. (2010). Quantitative chemical analysis. (Eighth Edition ed.). New York: W.H.

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Chance Boudreaux

ACKNOWLEGEMENTS: H. Z. Msimanga APPENDIX: The below example calculations are taken from trial 1:

(4)

0.09948 M   NaOH  0.02452 L  NaOH  1 mol   HCl  

1 mol   NaOH  0.09731 M   HCl 

(5)

0.05000  L  HCl   HCl 



0.02507  L  HCl   HCl excess



0.02507  L  HCl excess

0.02493  L HCl reacted 

(6)

0.09731 M   HCl  0.02493  L  HCl reacted  1 mol  Na2CO3 105.9885 MW  Na2CO3 2 mol  HCl 

(7)

0.1308 g   Na2CO3 0.3395 g  Total  Na  Na 2 CO3

0.4930 37.80

*100%  1 % RSD  RSD

0.1308 g   Na2CO3

*100 %  38.53 mass %  Na2CO3

(eq8 uses data from every trial; standard deviation over the mean of % mass Na₂CO₃)

(8)

