DETERMINATION OF THE PROPORTIONS OF SODIUM CARBONATE AND SODIUM HYDROXIDE IN A MIXTURE

DETERMINATION OF THE PROPORTIONS OF SODIUM CARBONATE AND SODIUM HYDROXIDE IN A MIXTURE

Objectives

1. To determine the mole proportions of sodium hydroxide and sodium carbonate in a mixture.
2. To determine the concentration of sodium hydroxide and sodium carbonate in the mixture in moles per litre.
3. To determine the concentration of sodium hydroxide and sodium carbonate in the mixture in grams per litre.

Introduction     

The neutralization of a strong alkali using a strong acid can be followed by the use of any indicator. Sodium hydrogen carbonate solution is slightly acidic to phenolphthalein, whereas sodium carbonate is alkaline. If an acid is added to a mixture of sodium hydroxide and sodium carbonate in solution, using phenolphthalein as the indicator, then the pink colour of the indicator is discharged when reaction I and II(a) are complete, as shown below. If methyl orange indicator is then added and a further quantity of the acid is added, then the amount of acid required will be that necessary to complete reaction II(b).

Reactions

• Phenolphthalein indicator

(I)            NaOH_(aq)  +  HCl_(aq)                     NaCl_(aq) + H₂O_(l)

(IIa)         Na₂CO_3(aq)  +  HCl_(aq)                  NaHCO_3(aq) + NaCl_(aq)

• Methyl orange indicator

(IIb)         NaHCO_3(aq) +  HCl_(aq)                       NaCl_(aq) + H₂O_(l) + CO_2(g)

The reaction between sodium carbonate and hydrochloric acid is a two-step reaction. In the first step (reaction IIa), an intermediate product, sodium hydrogen carbonate is formed. In the second step (reaction IIb), more hydrochloric acid is added to neutralize the sodium hydrogen carbonate formed in the first step.

The overall equation can be written as:

Na₂CO_3(aq)  +  2HCl_(aq)                   2NaCl_(aq) + H₂O_(l) + CO_2(g)

Procedure

0.2M hydrochloric acid was placed in the burette up to the 0cm³ mark. 25cm³ of the solution containing the two alkalis (sodium hydroxide and sodium carbonate) was placed in the conical flask using a pipette. Two drops of phenolphthalein indicator were added to the contents of the conical flask, where its colour turned pink. The acid was then run dropwise from the burette to the conical flask until the pink colour of the indicator just disappeared. The burette reading was recorded in table 1. A few drops of methyl orange indicator were then added to the contents of the conical flask. A further amount of acid was then run dropwise from the burette to the conical flask until the colour of the methyl orange indicator changed to orange. The burette reading was recorded in table 2. The above procedure was repeated three more times and the corresponding burette readings were recorded in tables 1 and 2 as shown below.

Results / Data analysis

Table of results.

Tables 1 and 2 below shows the various burette readings that were obtained in the experiment.

Table 1

Table 2

Calculations

Average volume of hydrochloric acid used in the first reaction (Table 1):

=

= 7.475 cm³

Average volume of hydrochloric acid used in second reaction (Table 2):

=

= 4.10 cm³

Questions

1. Volume of acid required to bring about reaction II(b).

The volume of the hydrochloric acid that was required to react with sodium hydrogen carbonate in reaction II(b) is that which was run from the burette after addition of methyl orange indicator.

The volume is, therefore, the average titre in table 2.

= 4.10cm³.

2. Volume of acid required to bring about reaction II(a), given that the number of moles of NaHCO₃is equal to the number of moles of Na₂CO₃.

In the second part of the reaction,

moles of HCl used in rxn II(b):

=

=

= 0.00082 moles.

moles of NaHCO₃ in rxn II(b)

HCl : NaHCO₃

1   :   1

= 0.00082 moles.

Since moles of NaHCO₃ = moles of Na₂CO₃

=0.00082 moles.

In the first part of the reaction,

moles of HCl used in rxn II(a)

HCl : NaHCO₃

1   :   1

= 0.00082 moles.

Volume of HCl used in rxn II(a)

=

=

= 4.10 cm³

3. Moles of Na₂CO₃and NaOH in a litre of the solution.

moles of Na₂CO₃ in 1 litre

0.00082mol                    25cm³

?                           1000cm³

=

= 0.0328 mol/l

volume of HCl that reacted with

NaOH in rxn I

= (7.475 – 4.1) cm³

= 3.375 cm³

moles of HCl that reacted with

NaOH in rxn I

=

=

=0.000675 moles.

moles of NaOH in 25cm³

NaOH : HCl

1   :   1

= 0.000675 moles.

moles of NaOH in 1 litre

0.000675mol                    25cm³

?                             1000cm³

4. Mass of Na₂CO₃and NaOH in 1 litre of the solution.

Mass of Na₂CO₃ in 1 litre

= mol/l x RMM

= 0.0328 x 106

= 3.4768 g/l

Mass of NaOH in 1 litre

= mol/l x RMM

= 0.027 x 40

= 1.08 g/l

=

= 0.027 mol/l

Discussion

The experiment above was a type of neutralization reaction where moles of an acidic solution (hydrochloric acid) were added to a fixed volume of a mixture of alkaline solutions (sodium hydroxide and sodium carbonate) until the equivalence point was reached; the point where the added titrant is chemically equivalent to the solution the conical flask.

Phenolphthalein indicator was added to the mixture of sodium hydroxide and sodium carbonate in the conical flask before the titration began, where its colour turned pink since the solution was alkaline. When hydrochloric acid was added to the mixture in the conical flask, it reacted with both sodium hydroxide and sodium carbonate. The last drop of the acid caused the colour of the indicator to turn from pink to colourless when the endpoint of the reaction was reached; the point where the indicator changes colour in the titration.

The reaction between hydrochloric acid and sodium carbonate resulted in the formation of sodium hydrogen carbonate, which is slightly acidic to phenolphthalein. Methyl orange indicator was added to the resulting solution to detect the endpoint of the second reaction. In the second reaction, more hydrochloric acid was added to the solution in the conical flask to neutralize the sodium hydrogen carbonate that was formed in reaction II(a). The colour of methyl orange changed to orange when the endpoint of the reaction was reached.

From the data collected in the experiment, the mole proportions of sodium hydroxide and sodium carbonate in the mixture, as well as their concentrations, were determined as shown in the calculations in the data analysis section.

However, the experiment may not have been carried out with absolute accuracy due to several errors such as:

• Parallax when reading the scale of the burette and the mark of the pipette.

• Wrong viewing of the meniscus of the liquids in the burette and pipette. (The viewing of the meniscus should be done perpendicular to the bottom of the meniscus).
• Use of contaminated apparatus such as burettes and pipettes leading to wrong mole values in calculations.

Conclusion

The moles of sodium carbonate in the mixture were found to be 0.00082 moles.

The moles of sodium hydroxide in the mixture were found to be 0.000675 moles.

The concentration of sodium carbonate in the mixture was found to be:

= 0.0328 mol/l

= 3.4768 g/l

The concentration of sodium hydroxide in the mixture was found to be:

= 0.027 mol/l

= 1.08g/l

References

   Books

Quantitative Inorganic Analysis by A. L. Vogel

Websites

»https://en.wikipedia.org/wiki/Neutralization_(chemistry)