Composition Analysis of NaOH and Na2CO3 Mixture

Objective

The objective of this experiment is to determine the composition of a mixture of solutions containing NaOH and Na2CO3 by employing the double indicator method.

Introduction

Consider a mixture of NaOH(aq) and Na2CO3(aq).

The reaction between HCl(aq) and Na2CO3(aq) occurs in two stages:

1. HCl(aq) + Na2CO3(aq) → NaHCO3(aq) + H2O(l) — (1)
2. HCl(aq) + NaHCO3(aq) → NaCl(aq) + CO2(g) + H2O(l) — (2)

However, the reaction between HCl(aq) and NaOH(aq) is a single-step process:

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) — (3)

In this titration, hydrochloric acid (HCl) is placed in the burette and added slowly, with constant swirling, to the mixture of NaOH(aq) and Na2CO3(aq).

At the equivalence point, the solution resulting from reaction (1) is alkaline, the solution from reaction (2) is acidic, and the solution from reaction (3) is neutral. Therefore, the pH curve for the entire titration should exhibit three breaks, corresponding to these three stages.

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Phenolphthalein is used to indicate reactions (1) and (3), while methyl orange is used to indicate reaction (2).

Procedure

1. Rinse all apparatus thoroughly with the corresponding solutions.
2. Pipette 25 cm³ of the mixture of NaOH(aq) and Na2CO3(aq) into a conical flask.
3. Fill the burette with standardized 0.1M hydrochloric acid (HCl).
4. Add 2 drops of phenolphthalein to the mixture.
5. Record the initial burette reading and slowly titrate until the solution changes from pink to colorless.
6. Record the final burette reading and add 3 drops of methyl orange.
7. Titrate again until the solution changes from yellow to red.
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8. Record the final burette reading to calculate the composition of the mixture of NaOH(aq) and Na2CO3(aq).

Data, Calculation, and Result

Phenolphthalein Indicator:

Mean titre (Phenolphthalein Indicator) = (25.00 + 24.95 + 24.95 + 25.00) / 4 = 24.98 cm³

Methyl Orange Indicator:

Mean titre (Methyl Orange Indicator) = (9.85 + 9.95 + 9.90 + 9.90) / 4 = 9.90 cm³

Number of moles of HCl (Methyl Orange Indicator) = (9.90 / 1000) x 0.1 = 9.9 x 10^-4 mol

Number of moles of NaHCO3 = 9.9 x 10^-4 mol

Number of moles of Na2CO3 = 9.9 x 10^-4 mol

Mass of Na2CO3 = 9.9 x 10^-4 x (23 x 2 + 12 + 16 x 3) = 0.105 g

Number of moles of HCl (Phenolphthalein Indicator) = (24.98 / 1000) x 0.1 = 2.498 x 10^-3 mol

Number of moles of the mixture of NaOH and Na2CO3 = 2.498 x 10^-3 mol

Number of moles of NaOH = 2.498 x 10^-3 - 9.9 x 10^-4 = 1.508 x 10^-3 mol

Mass of NaOH = 1.508 x 10^-3 x (23 + 16 + 1) = 0.0603 g

Mass ratio of Na2CO3 to NaOH = 0.105 g : 0.0603 g = 7 : 4

Therefore, the mixture contains approximately 64% Na2CO3 and 36% NaOH.

Discussion

The mean titre using the phenolphthalein indicator is 24.98 cm³, while the mean titre using the methyl orange indicator is 9.90 cm³. Based on these titration results, the mixture is composed of approximately 64% Na2CO3 and 36% NaOH.

Several sources of errors were identified during the experiment:

1. Inconsistent reading of the burettes.
2. General errors associated with titration, such as overshooting and arithmetic errors.
3. Methyl orange was added when the solution didn't become totally colorless.

Improvements:

1. Exercise great care during the addition of methyl orange to avoid premature color change.
2. Exercise great care during titration to avoid overshooting the endpoint, as it is not very sharp.

Questions

1. HCl is not a primary standard and needs to be standardized. Suggest a primary standard for the present experiment and briefly explain how to perform the standardization.

Answer: Anhydrous sodium carbonate (Na2CO3) can serve as a suitable primary standard for this experiment. To standardize HCl using Na2CO3, follow these steps:

1. Prepare a standard solution of Na2CO3 by accurately weighing a known quantity of anhydrous sodium carbonate and dissolving it in distilled water.
2. Add a few drops of phenolphthalein indicator to the Na2CO3 solution.
3. Titrate the Na2CO3 solution with the standardized HCl solution, recording the volume of HCl required to reach the endpoint when the pink color changes to colorless.
4. Calculate the molarity of HCl using the known mass of Na2CO3 and the volume of HCl used in the titration.

2. Write down the equation for the reaction involved before titration in the present experiment.

Answer: The reaction involved before titration in the present experiment is:

2NaOH + CO2 → Na2CO3 + H2O

3. Sketch a pH vs. volume of HCl curve and indicate the stages of color change of the indicators.

Answer: The pH vs. volume of HCl curve would show three distinct stages of color change for the indicators:

1. The first break corresponds to the transition from alkaline to neutral as indicated by phenolphthalein (pink to colorless).
2. The second break corresponds to the transition from acidic to neutral as indicated by methyl orange (yellow to red).
3. The third break corresponds to the completion of the reaction with NaOH (neutral).

4. Define the equivalence point and the end point of a titration. Why does one choose an indicator so that the two points coincide?

Answer: The equivalence point is the point in a titration at which the titrant (HCl) has been added in exactly the stoichiometric amount required to react with the substance being determined (the mixture of NaOH and Na2CO3). The end point, on the other hand, is the point at which the indicator changes color, indicating the completion of the reaction.

Choosing an indicator so that the two points coincide is essential to minimize titration errors. When the equivalence point and the end point coincide, it ensures that the reaction has reached its completion precisely when the indicator changes color. This alignment results in a more accurate determination of the unknown substance's concentration or composition.

5. Suggest another way to detect the equivalence point without the use of an indicator.

Answer: Another way to detect the equivalence point without the use of an indicator is to use a pH meter. Continuously monitor the pH of the solution during titration. The equivalence point will be reached when the pH levels off and remains relatively constant, indicating that the reaction is complete.

Conclusion

The analysis of the experimental data reveals that the mixture is composed of approximately 64% Na2CO3 and 36% NaOH.