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Concentrated hydchloric acid was titrated against a primary standard of sodium...

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nerdygirl-1996 | Student, Undergraduate | (Level 1) Honors

Posted July 22, 2013 at 3:27 AM via web

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Concentrated hydchloric acid was titrated against a primary standard of sodium carbonate and the following burette volume results were obtained:

 

TITRATION

Rough

1st Accurate

2nd Accurate

3rd Accurate

4th Accurate

Initial reading  (mL)

0

13.6

0

3.8

14.1

Final Reading (mL)

20

33

19.2

24.5

33.4

Titration volume (mL)

20-0 = 20

33-13.6= 19.4

19.2-0= 19.2

24.5-3.8= 20.1

33.4-14.1= 19.3

In each titration 20 ml of 500ml, 0.05mol/L sodium carbonate solution was pipetted into a conical flask and methyl orange was added. The hydrochloric acid was initially prepared as 500ml of approximatley 0.1 mol/L solution. The purpose of the experiment was to standardise the hydrochloric . Using the equation for this reaction calculate the number of moles of sodium crabonate used in each titration and the number of moles of hydrochloric acid used in the titration 

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llltkl | College Teacher | (Level 3) Valedictorian

Posted July 22, 2013 at 7:20 AM (Answer #1)

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The number of moles of a reagent in a certain volume of its solution is obtained by multiplying its molarity with the volume in litres.

Thus the number of moles of Na2CO3 (primary standard) used in each titration

=20*10^(-3)*0.05

=0.001

When methyl orange is used as indicator in the titration of Na2CO3 vs. HCl, replacement of both the sodium ions in Na2CO3 by H+ is recorded.

The strength of HCl is first determined by plugging in the average titre value in the titration equation, i.e. V_1*N_1=V_2*N_2 (Note that, N-terms indicate the strengths in normality, which is twice the molarity for a diacidic base like Na2CO3, but equal to molarity for a monobasic acid like HCl):

Here, among the accurate titration results, the third accurate is not acceptable because its value is discordant from the rest. So, the accepted average titre value is (19.4+19.2+19.3)/3=19.3 mL.

therefore, the strength of HCl solutionin molarity

=(20*0.05*2)/19.3=0.103627 (M)

Hence the number of moles of HCl used in each titration is as follows:

Obs. Schedule            Titre value      Number of moles of HCl used

----------------------   ------------   ----------------------------------

Rough                           20     20*0.103627*10^(-3)=0.002073

1st Accurate                 19.4   19.4*0.103627*10^(-3)=0.00201

2nd Accurate                19.2   19.2*0.103627*10^(-3)=0.00199

3rd Accurate                20.1    20.1*0.103627*10^(-3)=0.002083

4th Accurate                19.3    19.3*0.103627*10^(-3)=0.002

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