Average titrant used = (15.3+15.4+15.3)/3
=15 1/3cm3
Question
- What are the requirements for a compound to make a primary standard solution?
Standard solution is a chemical term which describes a of known .When preparing a primary standard solution, the chemical to be used for preparation of the solution should have several properties.
First, the chemical to be used should have high purity .The chemical with high purity can ensure the standard solution made also have relatively high purity .This can prevent the impurities in the solution from reacting with the chemical to be titrated (e.g. HCL (aq)) If impurities are present, side reactions may take place and other new chemicals, which is undesirable in that experiment, will be produced .Eventually, the result from the experiment will be inaccurate
Second, the chemical to be used should have high chemical stability. This ensures the standard solution will not react with other chemicals in the atmosphere or the surroundings (e.g. beaker, burette etc.).Otherwise, unexpected side reactions will be triggered which may also make the results of reaction as well as calculation inaccurate. Moreover, the chemicals used which are chemically unstable may react with any other chemicals and lead to danger unexpectedly.
Third, chemicals with low and are preferred to be used for preparing primary standard solution. The chemicals which show hygroscopic properties (e.g. concentrated sulphuric acid) will absorb any moisture in the air at anytime .This makes the calculation of molarity of that standard solution in the previous time become inaccurate and unpredictable as the molarity will change with the volume of moisture absorbed. Furthermore, a series of error will be made after the reactions and calculations. Similarly, the of chemicals refers to the loss of number of mole of certain chemicals. This makes the calculation and results obtained forth become inaccurate as well.
Fourth, the chemicals to be used should be highly soluble. For instance, in this experiment, Na2CO3 should be supposed to be highly soluble. If the solid is not completely dissolved, the calculation of the molarity of the standard solution will be in accurate as the weighed solid is not completely used in the titration. Thus, the number of moles of Na2CO3 I is less than the calculated results. Hence, the actual molarity of HCL (aq) cannot be accurately calculated due to serious of errors.
On the other hand, for handling and treat the chemical easily during and after the experiment, the chemicals used should be non-toxic. This is because, there will be likely a close contact between the chemists and the chemicals. If the chemicals used are toxic, the chemists will be probably poisoned by that chemicals as well as the standard solution produced. Furthermore, the chemicals used should be cheap and readily available for economical reasons. Another property is becoming more considerable nowadays, which is the eco-friendliness of that chemical. This ensures the standard solution after using can be disposed of without leading to a great impact to the environment. Otherwise, natural habitats may be damaged by the decomposition and side reaction of the standard solution after disposal.
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Calculate the molarity (in mol dm-3) of the given hydrochloric acid.
2HCL (aq) + Na2CO3 (aq) →2NaCl (aq) +CO2 (g) + H2O (l)
No. of mole of Na2CO3 reacted=1.2992/ (x2+ 12.0107+ x3)x 1/10
=0.0012398314mol
No of mole of HCl(aq)= 2x0.0012398314
=0.002479662867mol
Molarity of HCl(aq)= 0.002479662867/[(15 1/3)/1000]
=0.002479662867(23/1500)
=0.161717143
~0.162M
∴The molarity of given hydrochloric acid is 0.162M
- Is it necessary to rinse the pipette with the standard sodium carbonate? Explain.
Before titration, pipette should be washed with distilled or de-ionized water and then with the solution to be contained (Na2CO3 (aq)). After washing with distilled or de-ionized water ,there maybe water or impurities remained in the pipette. Any water or impurities remained inside the pipette , will dilute the standard sodium carbonate to be contained. In other words , the number of mole of Na2CO3 will be lowered and the Na2CO3 (aq) will be no longer standard. Thus, the molarity of the given HCL(aq) calculated will be inaccurate due to the titration with contaminated Na2CO3(aq). Therefore, pipette should also be rinsed by standard sodium carbonate before transferring the solution during the actual titration. By doing this step, impurities and water remained can be washed away and the number of mole of Na2CO3 in the pipette will be in accordance with the calculated data.
- Suggest one more method other than using acid-base indicator to detect the end-point of an acid-alkali titration.
Data logger with temperature sensor can also be used to detect the end-point of an acid-alkali titration. A data logger is an electronic device that records data over time or in relation to location either with a built in or or via external instruments and sensors.
There are many advantages of using data logger. Data loggers are generally small, battery powered, portable, and equipped with a microprocessor, internal memory for data storage, and sensors. Moreover, chemists can utilize software to activate the data logger and view and analyze the collected data. Thus , data can be automatically collected on a 24-hour basis. This allows for a comprehensive, accurate picture of the environmental conditions being monitored, such as the temperature of the solution in conical flask in this titration. Furthermore ,the cost of data loggers has been declining over the years as technology improves and costs are reduced. Simple single channel data loggers cost as little as $25US.
Fig.1
From fig. 1,at point 1, as neutralization is an exothermic process, temperature rises during the reaction. At point 2 ,the most significant nature of the neutralization is that the temperature of the solution reacted reaches a maximum at the end-point of the neutralization. At point 3, after neutralization is completed, excess HCL(aq) added cools down the solution. Thus, the temperature of solution in conical flask drops. Thus, the end point of acid-alkali titration can be accurately detected by the temperature sensor connecting to the data logger.
- Discuss the sources error of this experiment
There are many sources of errors in this experiment.
First of all, chemists may misjudge the color of the indicator(e.g. Methyl orange changes from yellow to red) near the end point . This is probably the most common one. Not only color change is sometimes very delicate and slow, but different people have different sensitivity to colors. Thus, excess or insufficient volume of HCL(aq) may be added when the colour of solution is not significant to show the end-point.
Second, chemists may misread the volume of HCL(aq) contained in burette. This can be a parallax problem (when someone reads the volume looking at inadequate angle), or error in counting graduation marks. Moreover, in different lighting conditions, upper and lower value of the volume may be misread.
Third, error may be made by using contaminated solutions .For example, two different solutions (HCL(aq)/ Na2CO3 (aq ))are transferred using the same pipette or beaker and pipette is not rinsed with distilled water in between.
Fourth, diluted titrant and diluted titrated solution may be accidentally used in the titration if the burette or pipette are not rinsed with solution to be contained or transferred after being rinsed with distilled water. Consequently, titrant is slightly diluted.
Fifth, using solutions of wrong concentration .This can be due to incorrect standardization, error in copying the concentration, contamination of the bottle content, titrant decomposition, solution being kept in open bottle and partially evaporated and so on.
Sixth, wrong amount of indicator may be used . It is suggested that same volume of indicator, say methyl orange, should be used in every single titration. Otherwise, excess single color indicator added can shift the end point.
Seventh, error is likely to be made by filling burette properly .If there is an air lock in the burette stopcock it can block the flow of the titrant, but it can also at some moment flow with the titrant; after that we have no idea what was the real volume of solution used.
Eighth, leaking burette will be a disaster of making error. Sometimes burettes leak slowly enough to allow titration, but will loose several tenths of milliliter if left for several minutes. Thus, excess volume of HCL(aq) may be added without awareness. As a result, the actual volume of the solution added will be unknown.
Ninth, solid or liquid are not all transferred properly - it may happen that part of the solid sodium carbonate was left on the beaker when dissolving or it was simply lost. It is also common to forget to rinse walls of the beaker and conical flask after solution and indicator were transferred as droplets always adhere on the flask wall and was not rinsed with distilled or de-ionized water.
Tenth, excess volume of liquid will be transferred by blowing pipette for example, or by incorrectly leveling meniscus with the mark on the single volume pipette. It should be noted that last drop of solution should never be blown as this volume of the droplet is counted by the manufacturer of pipette.
Eleventh, titrating at wrong temperature can also be an error as some indicators are sensitive to temperature changes and some reactions need correct temperature range to keep stoichiometry
Twelfth, the colour change of indicator does not necessarily indicate the equivalence point. Although the end point is reasonably close to the equivalence point , there is still minor difference in volume. Thus, the volume of HCL(aq) added is not exactly the actual volume required for completely neutralizing Na2CO3 (aq).
Last but not least, solution may be last due to too vigorous swirl . This can end in liquid splashing from the conical flask before the end point had been reached.
Conclusion
To conclude, the molarity of the given hydrochloric acid is tested and calculated to be 0.162M.
Throughout the titration, it should be noted that chemists should find a correct and suitable chemicals to make primary standard solution. Moreover, it is necessary to rinse the pipette with the solution to be contained or transferred. Furthermore, chemists can use other methods ,such as using data logger with temperature sensor ,to detect the end-point of acid-alkali titration. Furthermore, chemists should aware of the potential errors may be made, in order to decide appropriate measures to reduce the error. By implementing the measures above, the results of titration can be more accurately obtained.
Reference
Yahoo!Mini
HKCEE Chemistry Notes Section IV by Ms Sin W L
F.6 Chemistry Notes Section I by Ms Sin W L
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