B. Using Standard Sodium Carbonate to titrate Sulphuric Acid
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Dilution of H2SO4: 25cm3 of the given bench sulphuric acid in a beaker was pipetted into a 250.0 cm3 volumetric flask. The beaker was rinsed with a small amount of distilled water and the rinse was transferred into the volumetric flask. The step of rinsing was repeated twice. Distilled water was added to the volumetric flask carefully. The volumetric flask was stoppered tightly. It was turned upside down and shook several times in order to mix the sulphuric acid with water thoroughly.
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The following titration set-up was constructed as shown in the diagram.
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The filter funnel used in Part A was rinsed with distilled water. The stopcock of the burette was closed. About 10 cm3 of distilled water was added into the burette through the funnel. The funnel was removed. The burette was rotated horizontally to rinse its inner wall. The stopcock was opened and the water was allowed to run out into the sink. This step was repeated once.
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Step 3 was followed to rinse the burette with several cm3 of the given sulphuric acid (but not with water). This step was repeated once.
- The stopcock of the burette was closed. The given sulphuric acid was poured through the funnel into the burette until the liquid level was near zero. The stopcock of burette was opened to allow the titrant to fill up the tip and wash the bubbles out.
- The conical flasks was rinsed with distilled water.
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A pipette filler and a 25.0 cm3 pipette was taken. The pipette filler was pressed and then its tip was put into the top end of the pipette tightly. The pipette and the pipette filler were kept connected together. The tip of the pipette was inserted into a beaker of distilled water. The air pressure in the pipette filler was released to suck several cm3 of water. The pipette filler was removed and the top end of the pipette was closed with a forefinger immediately. The tip of the pipette was removed from the water. The pipette was rotated horizontally to rinse its inner side. Then, water was allowed to run out to the sink. This step once was repeated once. Finally the outside wall of the pipette was wiped with tissue. About 40cm3 of the standard sodium carbonate solution was poured into another clean 100cm3 beaker.
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Step 7 was followed; the pipette was rinsed with the standard sodium carbonate solution in the 100cm3 beaker (but not with water). This step was repeated once.
- The pipette filler was used to fill the pipette with the standard sodium carbonate solution until the solution was several cm above the graduated mark.
- The pipette filler was removed and the top end of the pipette was closed with a forefinger immediately. The tip of pipette was removed from the standard sodium carbonate solution (better to wipe the tip with tissue quickly). The liquid level was adjusted with the forefinger carefully until it just touched the graduated mark.
- The pipette was inserted into a conical flask. The forefinger was removed to allow the standard sodium carbonate to drain into the flask. When the drainage stopped, the tip of pipette was allowed to touch onto the inner wall of conical flask to allow the remaining droplet to flow out.
- 2 drops of methyl orange indicator was added to the conical flask.
- The initial burette reading was recorded in the table provided. The reading should be accurate to 2 decimal places.
- The tip of burette was inserted into the mouth of conical flask containing the standard sodium carbonate. Titration was started by opening the stopcock of the burette to allow sulphuric acid to drain into the flask. During the process of titration, the conical flask was swirled continuously to mix the solutions. When the solution in the conical flask just changed from yellow to orange, the stopcock of the burette was closed immediately.
- The final burette reading was recorded in the table provided. This was a trial titration to estimate the volume of hydrochloric acid required. The volume of sulphuric acid added in titration was calculated.
- The given sulphuric acid was added to the burette through a filter funnel if the volume remained was not enough to carry out another titration.
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Steps 9-16 were repeated to obtain 3 more sets of consistent results (difference of results within 0.10 cm3). However, draining of sulphuric acid was stopped at about 3 cm3 less than the estimated value. Then sulphuric acid was added drop by drop until the reaction mixture in conical flask just changed from yellow to orange.
Results
Observation
Change of solution from yellow to orange
Data
Average volume of hydrochloric acid reacted = (14.2+14.3x2)/3 =14.267cm3
Calculations
Na2CO3 (aq) + H2SO4(aq) Na2SO4 (aq)+ CO2 (g)+ H2O(l)
Molar mass of Na2CO3=106g/mol
No. of mole of Na2CO3: mass/molar mass
=1.3g/106g/mol
=0.01226mol
Concentration of standard solution= 0.01226mol x 1000/250
=0.049mol/dm3
25cm3 of this standard solution is then titrated against H2SO4 and from the above equation,
1 mol Na2CO3 neutralizes 1 mol H2SO4.
Amount of Na2CO3 used in the titration process
=25/1000 x 0.049mol/dm3 =1.23 x 10-3mol
= Amount of H2SO4 used
Thus, the molarity of H2SO4 after dilution :
1.23 x 10-3 mol ÷14.267/1000 dm3
= 0.08596 M
Molarity of H2SO4 before dilution=0.08596 M x 10
=0.8596 M
- Answer to Study Questions
- What does the phrase ‘weigh out accurately about 1.3 g’ imply?
-It implies that we have to weigh out 1.3 g of sodium carbonate with an accurate electronic balance using the method of measurement by difference. However, it is enough for us to estimate up to 2 decimal numbers when the figure is almost stable shown on the balance.
- For what reasons should the weighing process be carried out as quickly as possible if filter paper instead of weighing bottle is to used?
-It is to avoid absorption of moisture from the atmosphere to the filter paper
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A student said that he was supposed to dissolve the weighed sodium carbonate sample in 250cm3 of distilled water in a volumetric flask. Is there any mistake in his saying? Explain.
-The student was incorrect in saying that. He was supposed to dissolve the weighed sodium carbonate in a beaker by less than 100 cm3 of distilled water but not in a volumetric flask. Also, we dilute the sodium carbonate solution instead of dissolving it in a volumetric flask.
- Does it spoil your experiment if you happen to add extra drops of bench dilute sulphuric acid in the dilution step? Explain your answer.
-Yes, it does because we ought to keep the number of moles of solute unchanged in the dilution step. Adding more sulphuric acid does increase the number of moles of the acid, hence, it is improper to do so.
- ‘It is better to prepare a more concentrated solution of sodium carbonate so that bench dilute sulphuric acid can be titrated with it without dilution procedure.’ Comment on this statement.
-This statement is wrong because the dilution of dilute sulphuric acid is not related to the concentration of sodium carbonate solution. The bench sulphuric acid is further diluted to prepare a solution of approximate concentration first.
Conclusion:
To conclude, the concentration of the given bench sulphuric acid is 0.8596 M .If we want to determine the concentration of unknown concentration either acid or base, we can do the titration process with applying the correct technique in titration. If we do not use the correct technique, there must have some error in our result.
Acknowledgement: T. M. Leung & C. C. Lee, Phy. Chem. I (4th Ed.), p.50-59 “Titration”, B Earl & LDR Wilford, Introduction to Advance Chemistry, p.39-43
Discussion:
- Choice of Standard solution:
In the experiment, we chose to use anhydrous sodium carbonate as the primary standard because it is (1) available in a pure form,
(2) readily soluble,
(3) not volatile---if it was as NaOH, then losses would occur during the weighing out of the substance,
(4) unable to absorb CO2 or water from the atmosphere, (5)stable, that is, not decompose easily as solid or in solution ,
(6) having a fairly high molar mass, so that the weighing error in using a given number of moles of the solid is reduced
2. Choice of indicator
Methyl orange instead of phenolphthalein was used in the titration since H2SO4 is a strong acid whereas Na2CO3 is a weak alkali. An ideal indicator for a titration is one whose end point matches the equivalence point, which is the point at which equivalent amounts of the acid and alkali have reacted. Hence, according to the below graph, a sharp change in pH takes place at the equivalence points where the acid and alkali just neutralize each other, and a sharp colour change from yellow to orange can be detected. Methyl orange should be used.
3. Errors in volumetric analysis
- Electronic balance: The balance used was read to 0.01g ,so
% error=0.01g/mass measured(g) × 100
=0.01g/1.3g × 100
=0.77%
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Pipette: The pipette used can be read to 0.1cm3. The error due to the use of a pipette would depend on its volume. The error associated with using a 25 cm3 pipette is:
%error= 0.1cm3/25cm3 × 100
= 0.40 %
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Burette: The burette used was read to 0.1cm3. The error from this piece of equipment depends on the amount of solution titrated to reach the end-point. The smaller the volume titrated, the larger the error. In the practical the error is:
%error=0.1cm3/14.267cm3 × 100
=0.70%
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Volumetric flask: Again the error depends on the size of flask used. Since a 250cm3 flask was used to be read to 0.1 cm3, the error is:
%error=0.1cm3/250cm3 × 100
=0.04%
The overall error of using the four pieces of apparatus above:
0.77% + 0.40% + 0.70% + 0.04% = 1.91 %
- Other errors:
1. When I was making up the solution, the sodium carbonate powder may not dissolve well and the solution may not mix well.
2. Before the titration, I need to use distilled water to wash the pipette and burette. Then use chemical that need to contain in the pipette and burette to wash them.
3. During these processes, some distilled water may leave on the surface of them.
4. After each titration, I need to wash the conical flask again. Therefore, some chemicals may leave on the surface of the conical flask.
5. When I was pouring the dissolved sodium carbonate into the volumetric flask, I may carelessly pipetted 25 cm3 of it instead of pouring the whole beaker of solution into the volumetric flask. Hence, the no of moles of Na2CO3 was apparently reduced and the molarity of sulphuric acid was underestimated.
Those cases I mention above may affect the result of the experiment. Therefore, my result (the concentration of sulphuric acid and the relative molecular mass of sodium carbonate) may not be very accurate.
- Precautions
- Remove the funnel above the burette to prevent an ‘air lock’ during titration
- The tip of the burette should be brought closer to the solution in the conical flask, to prevent excessive splashing of un-neutralized droplets all over the sides of the conical flask.
6. Rinsing the conical flasks provided with distilled water does not spoil the titration because the number of moles of solute remains unchanged after water is added.
~END OF LAB REPORT~