Chance BoudreauxDETERMINATION OF Na ₂ CO ₃ 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)CO

32−+H3O+⃗ HCO3−

+H2 O

(2)CO

32−+2 H 3O+ ⃗ HCO3−

+ H2O

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

indicator methyl orange.

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Chance BoudreauxIn 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 by the use of 0.09731M HCl and 0.09948M

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 4Na₂CO₃

(g) 0.3395 0.3579 0.3049 0.3049HCl (mL) 50.00 50.00 50.00 50.00H₂O (mL) 25.00 25.00 25.00 25.00

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)Na2 CO3+2 HCl⃗ 2NaCl+CO2+H2O

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 HCl1 mol NaOH

|M HCl

=L HClexcess

(5) L HCl−L HCl excess=L HClreacted

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Chance Boudreaux

(6)

M HCl|L HCl reacted|

1 mol Na2 CO3

2 mol HCl|

MW Na2 CO3=g Na2 CO3

(MW Na₂CO₃= 105.9885 g/mol)

(7)

g Na2CO3|gTotalNa2 CO3

∗100 %=¿mass % CO3 ¿

(8)

Sx

x¿ ∗100 %=% RSD

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:

This experiment is created to quantify CO₃ in an unknown purity of Na₂CO₃. This is done by a

means of a back titration technique, due to two possible reactions between H₃O⁺ and CO₃²¯. This

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Chance Boudreauxback titration is done by adding a known excess amount of standardized HCl to a weighted

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 t-

test. 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.

Freeman and Company.

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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 HCl1mol NaOH

|0 .09731 M HCl

=0 .02507 LHCl excess

(5) 0 .05000 L HCl−0. 02507 L HClexcess=0. 02493 L HCl reacted

(6)

0 .09731 M HCl|0 . 02493 L HClreacted|

1mol Na2 CO3

2mol HCl|105 . 9885 MW Na2 CO3 =0. 1308 g Na2 CO3

(7)

0 .1308 g Na2CO3|0 .3395 g TotalNa2CO 3

∗100 %=¿38 .53 mass % Na 2CO3 ¿

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

(8)

0 . 4930

37 . 80∗100 %=1% RSD

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