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Objective: The purpose of this experiment is to determine the composition of a mixture of solutions of NaOH and Na2CO3 by double indicator method.

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Introduction

Experiment 4: Acid-base Titration using Method of Double Indicators Date: 2010/10/7 I. Objective: The purpose of this experiment is to determine the composition of a mixture of solutions of NaOH and Na2CO3 by double indicator method. II. Introduction: Consider a mixture of NaOH(aq) and Na2CO3(aq). Reaction between HCl(aq) and Na2CO3(aq) takes place in two stages: HCl(aq) + Na2CO3(aq) � NaHCO3(aq) + H2O(l) -------------------------(1) HCl(aq) + NaHCO3(aq) � NaCl(aq) + CO2(g) + H2O(l) -----------------(2) While that between HCl(aq) and NaOH(aq) completes in only one step: HCl(aq) + NaOH(aq) � NaCl(aq) + H2O(l) -----------------------------(3) The hydrochloric acid is placed in the in the burette and is added, slowly and with constant swirling, to the mixture of NaOH(aq) and Na2CO3(aq). Solution mixture of reaction (1) at the equivalence point is alkaline, while that of reaction (2) ...read more.

Middle

could be calculated. IV. Data, Calculation and Result Phenophthalein indicator: Trial 1st 2nd 3rd 4th Final burette reading(cm3) 30.50 27.50 26.95 26.20 27.40 Initial burette reading(cm3) 5.40 2.50 2.00 1.25 2.40 Titre (cm3) 25.10 25.00 24.95 24.95 25.00 Mean titre = (25.00+24.95+24.95+25.00)/4 = 24.98 cm3 Methyl orange indicator: Trial 1st 2nd 3rd 4th Final burette reading(cm3) 40.50 37.35 36.90 36.10 37.30 Initial burette reading(cm3) 30.50 27.50 26.95 26.20 27.40 Titre (cm3) 10.00 9.85 9.95 9.90 9.90 Mean titre = (9.85+9.95+9.90+9.90)/4 = 9.90 cm3 Number of mole of HCl(Methyl orange indicator)= 9.90/1000X0.1 = 9.9X10-4mol Number of mole of NaHCO3= 9.9X10-4mol Number of mole of Na2CO3= 9.9X10-4mol Mass of Na2CO3= 9.9X10-4 X(23X2+12+16X3) = 0.105g Number of mole of HCl(Phenophthalein indicator)=24.98/1000X0.1 =2.498X10-3mol Number of mole of the mixture of NaOH and Na2CO3=2.498X10-3 mol Number of mole of NaOH=2.498X10-3 - 9.9X10-4 =1.508 X10-3 mol Mass of NaOH=1.508 X10-3X(23+16+1)=0.0603g Mass ratio of Na2CO3 to NaOH=0.105:0.0603 =7:4 Therefore, the mixture contains approximately 64% Na2CO3 and 36% NaOH V. ...read more.

Conclusion

The molarity of the given hydrochloric acid can be found by titrating it against the standard sodium carbonate solution prepared. The end-point is marked by using methyl orange as indicator. 2. Write down the equation for the reaction involved before titration in the present experiment. Ans: 2NaOH +CO2 �Na2CO3 3. Sketch a pH vs volume of HCl curve, indicate the stages of colour change of the indicators. 4. Define the equivalence point and the end point of a titration. Why does one choose an indicator so that the two points coincide? Ans: Equivalence point is the point at which titrant has been added to react exactly with the substance being determined while end point is the point at which the indicator changes colour. Choosing an indicator that the two points coincide helps to minimize titration error. 5. Suggest another way to detect the equivalence point without the use of indicator. Ans: pH meter. VII. Conclusion The mixture is composed of approximately 64% Na2CO3 and 36% NaOH ...read more.

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