Determine the concentration of a given bench hydrochloric acid of about 0.1M.
2. Chemicals and Apparatus
* Standard Solution of Sodium Carbonate (Na2CO3)
* Hydrochloric Acid of about 0.1M
* Methyl Orange Indicator Solution
* Conical Flask
* Pipette Filler
* 25 mL Pipette
* 250 cm3 Beaker
* 250 cm3 Volumetric Flask
* Filter Funnel
* Plastic Washbottle
* Glass Rod
* Deionized water
Hydrochloric acid is a strong acid and can react with weak base sodium carbonate according to the equation below:
2HCl + Na2CO3 –> 2NaCl + H2O + CO2
To determine the concentration of the bench hydrochloric acid, which is about 1 M, the acid is titrated against a standard sodium carbonate solution.
Methyl orange is used to indicate the completion of the reaction, as its colour will change from yellow (in alkaline solution) to pale orange when the equivalence point (also known as stoichiometric point) has reached.
* Rinse the burette with water and then with the diluted hydrochloric acid.
* Fill the burette with the acid to rinse and fill the tip.
* Record the initial reading in the “Trial” column of the Results Table, and round it off to the nearest 0.05 cm3, which is approximately one drop.
* Use a pipette filler to rinse the pipette with water and the sodium carbonate solution. 25.0 cm3 of sodium carbonate solution is then carefully transferred to a clean 250 cm3 conical flask. Note that this first flask is usually taken as a trial run as the end-point is probably overshot.
* Run the diluted hydrochloric acid from the burette into the flask. The flask is swirled until the colour of solution has just changed from yellow to pale orange.
The final burette reading was recorded.
* Refill the burette with the acid, and take down the initial reading again.
* Transfer another 25.0 cm3 of the standard solution to a clean conical flask with 3 drops of the methyl orange indicator solution added.
* Titrate the solution to the end-point carefully. Add the alkali drop by drop into the flask when the end-point is near and the colour is about to change from yellow to pale orange.
* Repeat the above 3 steps for at least 3 times, with the readings recorded down in the Results Table.
* Empty the burette and wash it thoroughly immediately right after the titration.
* Results Table
Final Burette Reading (cm3)
Initial Burette Reading (cm3)
* Average Titre Used = = 27.25 cm3
* 2HCl + Na2CO3 –> 2NaCl + H2O + CO2
Mole Ratio of HCl to Na2CO3 = 2:1
Let M1 and V1 be the Molarity and Volume of HCl
Let M2 and V2 be the Molarity and Volume of Na2CO3
?M1 = = 0.1028 M
* The molarity of the diluted hydrochloric acid solution is 0.103 M.
* The molarity of the bench hydrochloric acid solution is 1.030 M.
6. Questions and Answers
* Dilution of the bench hydrochloric acid before titration
Diluting the acid by ten times could effectively reduce the time needed for the experiment. This is because the diluted acid would have lower molarity, thus less amount of the acid is needed to run through the burette in order to reach the end-point. Moreover, this helped to save the chemicals and prevent wastage as less solution and acid is being used during the titration. Also, as the acid is diluted by ten times, the error in the accuracy of readings is highly reduced at the same time.
As stated above, 25 cm3 of hydrochloric acid could first be transferred into a 250 cm3 volumetric flask with a pipette. Deionized water is then added until the meniscus had reached the graduation mark precisely. The volume of the solution is now increased by 10 times, from 25 cm3 to 250 cm3. As there is no addition of solutes, the solution is diluted exactly by 10 times.
* Effect of errors on non-rinsed burette with diluted hydrochloric acid solution
Having the burette rinsed with deionized water, some water droplets may easily adhere onto the wall of the burette. The water remained may dilute the acid running into the burette, and more diluted acid may be needed to neutralize the sodium carbonate solution (V1).
M1 (?) V1 (?) = (Molar Ratio) (M2V2) = constant
The number of moles of sodium carbonate solution, M2V2, is calculated based on the data on the molarity and volume of the solution, which is considered as a known constant. The equation above demonstrates that an increase in V1 will result in a decrease in M1, the molarity of the acid. This shows that the calculated concentration of the acid will be lowered.
Running the diluted hydrochloric acid solution into the burette would wash away these water droplets together, and this could ensure a more accurate and precise calculation on the concentration of the acid.
* Effect of errors on non-rinsed pipette with sodium carbonate solution
Again, water droplets may still remain on the wall of the pipette after the rinsing process. Diluting the solution in the pipette, the molarity of the sodium carbonate solution will decrease (M2). Less hydrochloric acid will be required to neutralize the sodium carbonate solution as the molarity is decreased, and the calculated concentration will be enlarged.
On the other hand, rinsing the pipette with the solution would clean the inner wall of the pipette thoroughly and wash away the droplets, this contribute to yielding a more accurate calculation of the unknown concentration.
* Effect of errors on non-filled tip of burette before titration
The reading on the burette actually includes the liquid in the tip of the burette. If the tip is not filled before titration begins, extra liquid is needed in order to fill up the tip after the titration. This would result in a larger reading and the calculated concentration will be smaller.
M1 (?) V1 (?) = constant
* Effect of errors on conical flasks containing distilled water before the addition of sodium carbonate
There will have no effect to the experimental result. The concentration of the original sodium carbonate has already been accurately measured before transferring to the flask. An addition of water, which is neutral, will not alter the amount of acid needed to neutralize the solution.
* Reasons for using methyl orange as the indicator
The choice of indicator in a given titration depends on the strength of the acid and the alkali involved. Litmus paper and universal indicator are not used because they do not give sharp colour changes. In this experiment, since hydrochloric acid is a strong acid whereas sodium carbonate is a weak alkali, generally methyl orange is taken for detection of end point, as the volume required for reaching the end point and the equivalence point is about the same. In fact, as the resultant solution after the titration is a weak acid, methyl orange is very applicable to indicate the sharp colour change from yellow to pale orange. As a matter of fact, methyl orange changes from red to yellow over the pH range from 2.7 to 4.7, and it appears orange when the pH value reaches 3.7. Below shows the relationship between the pH scale and the indicators.
* Difference between the end-point and equivalence point in a titration
It is a experimental point where a sharp and obvious physical change indicates the completion of the reaction. This point is easily detectable, by colour change, electrical conductivity change, pH value change and temperature change.
It is a point where the acid has just exactly react with the base, which means that an exactly equivalent amount of titrate has been added to the sample. Neither acid nor base is in excess. It is not necessary with neutral pH, however. This point cannot be detected by any simple means.
The titration curve shows the changes in pH in the process of titration. It demonstrates the difference between the end point and equivalence point:
Fig 2.1 Graph showing the relationship between volume of acid and pH value
There is several types of flasks on the laboratory bench. Conical flasks are used, however, because there can stand on the bench without support. The lower centre of gravity also prevents the flasks from easily overturned. Also, the small mouth of a conical flask can avoid splashing out of solution during swirling in titration. Unlike flat-bottomed flasks and round-bottomed flasks, the conical flasks are much suitable for this experiment.
Note that it is not necessary to dry and wash the conical flask with the reagent after cleaning it with water before titration. This is because the amount of the reagent inside the flask has been accurately measured by the pipette. Adding water, which is neutral in nature, will not alter the experimental result of the setup.
It is a laboratory apparatus used in quantitative chemical analysis to measure the volume of a liquid. With a graduated glass tube and a stopcock at one the bottom, the precise volume of the liquid dispensed can be determined by reading the graduations marked on the glass tube at the liquid level before and after dispensing it. This is far more accurate and convenient than using measuring cylinder or other apparatus.
Filling up the burette with acid
The burette should be filled up with acid rather than alkali, as the alkaline solution may attack the glass of burette. Not only would it result in inaccurate calculation of the concentration, it would also cause irreversible damage to the burette.
Usage of pipette
A pipette precisely deliver a specific volume (25 cm3) of solution. With the pipette filler, solution is sucked up to the graduation mark. Note that when the liquid stops dripping out into another container, touch the bottom of the tilted container with the tip of pipette for a few seconds. This ensures that the volume transferred is accurate. Note that the small drop of liquid remaining in the pipette would not affect the accuracy of the volume, as the design of the pipette has allowed this when it is calibrated.
Usage of burette
A burette must be air tight before the titration begins. Whack the side of the burette tip while solution is flowing to remove any air bubbles present. If an air bubble is present during a titration, volume readings may be in error. Also, to ensure that the burette is in good condition, check for solution on the tip to see if your burette is leaking. On the other hand, a washbottle should be used to rinse the tip of the burette as the acid is flowing to ensure better results.
Application of filter funnel
A filter funnel is commonly used for addition of liquid. Note that the last drop of acid stuck on the tip of the funnel must be delivered to the burette by letting the tip get in contact with the inner wall of the burette. Or else, the meniscus may alter a bit and this may also affect the calculation on the volume used for neutralization. Furthermore, one must make sure that the funnel is removed from the burette before the titration begins.
Titrating the acid with alkaline solution
During titration, acid is to be added slowly to the sodium carbonate solution. This must be done slowly and carefully so that it could avoid the solution from splashing out of the conical flask which may result in inaccuracy of calculation. Moreover, the inner wall of the conical flask should be rinsed with deionized water in plastic washbottle during the process. This measure could ensure that no droplets of the solution are adhered on the wall of the flask which may, again, affect the results.
* Experimental Errors
Acidic carbon dioxide
Although the mouth of the conical flask is small, it could not help to prevent the carbon dioxide in air, which is acidic in nature, from dissolving into the solution and affect the pH value.
Identification of end point
Since the colour between yellow and pale orange, as well as pale orange and red is so close, it might be a bit difficult to distinguish the right timing in which the end point has reached. Practising more may help to identify the colour sharply, but adding one or two drops of indicator solution may also help to recognize the colour change better.
Difference between end point and equivalence point
The difference between the end point and the equivalence point is the titration error, which is kept as small as possible by the proper choice of an end point signal and a method for detecting it. Methyl orange is used to determine the end point in this experiment, and though attempts have been made to minimize the difference, there is still a small error in it.
Few colourless bubbles are evolved during the titration process. The colour of the solution changed from yellow to pale orange at an instance. There is also a positive heat change during so.
The concept of neutralization is widely used and practiced in daily lives. It helped to neutralize the acidity of soil, due to acid rain and usage of acidic fertilizers, by adding limestone (calcium carbonate). Intake of Milk of Magnesia to alleviate the pain in stomach is also another common use. In addition to this, neutralization also helped to treat industrial waste before disposal and to produce fertilizer.
Hydrochloric acid is present in the stomach which acts as a sterilizer. It kills the germs which entered our body with the food bolus through the esophagus. However, concentrated hydrochloric acid is highly corrosive and must be handle in great care.
Commonly known that sodium chloride is table salt which act as food flavouring, it is an essential part of the diet of both humans and animals and is a part of most animal fluids. It aids digestion by providing chlorine for hydrochloric acid, a small but essential part of human digestive fluid. It is also used in other fields such as being chlorine bleach and food preservatives. A solution of sodium chloride and sodium hypochlorite is mixed to form bleaching solution which could whiten objects permanently by oxidation.
However, the major use of sodium chloride is as a raw material for the production of chlorine, sodium metal, and sodium hydroxide. Historically, salt has been used as money.
Besides, Chinese had already been using the method of salting to preserve food in the old days when refrigerator is not yet invented. The highly concentrated sodium chloride solution draw out all the water of microbes adhered to the food, and so the microbes become dehydrated and die in the end.
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