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AIMS
1.To understand the concept of titrimetric analysis.
2.To learn how to prepare hydrochloric acid solution and sodium hydroxide solution.
3.Practice on the basic operation of titration.
A “titration”is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known analyte.Titrations belong to a class of analytical techniques known as “volumetric analysis”.Since volumes can be precisely delivered and measured using standard laboratory equipment,titration techniques can yield both accurate and precise measurements if care is taken by the analyst.Accuracy is defined as the closeness of a result(usually the average of several measurements)to a known accepted value.Precision relates to the closeness of the measurements themselves.In any analytical experiment,one strives for both accuracy and precision to validate the results.
In this experiment you will be graded on both your accuracy(how close you are to the actual value)and the precision of your results(how reproducible they are).
In a titration an analyst titrates a solution of unknown concentration with a solution of known concentration (a standard solution).The stoichiometry of the reaction between the standard and the analyte is known.
Using a calibrated “burette”to add the standard solution,it is possible to determine with accuracy the amount of analyte present in the unknown solution whenthe titration endpoint is reached.The endpoint of a titration is the point at which the titration is complete.The endpoint is generally signaled by an indicator(see below) that is added to the analyte solution.This is ideally the same volume or very close to as the equivalence point—the volume of added titrant at which the number of moles of titrant is equal to the number of moles of analyte,or some multiple thereof.In the classic strong acid-strong base titration,the endpoint of a titration is the point at which the pH of the reactant is just about equal to 7,and often when the solution takes on a persisting solid color as in the pink of phenolphthalein indicator usedin this experiment.
At the equivalence of a titration,the moles of the analyte are calculated which mayyield quantitative information such as concentration(mol·L -1 )or molar mass(g·mol- 1 ).
The ability to perform a titration experiment well is a crucial skill that all general chemistry students must master.You will be conducting a titration for the next lab and,since most of you have yet to perform such experiments,we are offering you a chance to practice beforehand.Recall,as they say,“practice makes perfect.”
The goal of this experiment will be to practice and perfect your volumetric techniques using pipettes and a burette.Your lab instructor will demonstrate proper pipet and titration techniques.
Example:the titration of 20 mL of a 0.100 0 mol·L -1 strong monoprotic acid HCl with 20 mL of a 0.100 0 mol·L -1 strong base NaOH.
Figure 2-8
Reaction:HCl＋NaOH NaCl＋H 2 O,so the moles of HCl is equal to the moles of NaOH,( cV ) HCl ＝ n (HCl)＝ n (NaOH)＝( cV ) NaOH
● Analytical balance,250 mL volumetric flasks,50 mL burettes,250 mL conical beakers,25 mL volumetric pipettes,25 mL beakers,25 mL volumetric pipettes,droppers,glass rods,rubber bulbs
● Concentrated hydrochloric acid(HCl),sodium hydroxide(NaOH),indicator:0.1％ methyl orange,0.1％phenolphthalein solution,distilled and deionized water
Using a volumetric pipet and a bulb,pipet 4.2 mL of about 6 M concentrated HCl(aq)into a 250 mL volumetric flask.(Record the exact concentration(mol/L) written on the container).
Weigh about 1.0 g NaOH,dissolve it completely in beaker,then transfer into a 250 mL volumetric flask.
Retrieve a burette from the wooden case in your lab.Inspect it carefully to see that the glass end to the stopcock is not plugged and that the stopcock does not leak.Empty the burette into the sink(save the stopper)and rinse your burette with a small amount of deionized water(～10 mL)three times.
Close the stopcock and rinse your burette with a small amount of the prepared HCl solution(～5 mL)three times.On the third rinse,allow the acid to drain from the stopcock.You can use one of your large beakers to collect waste.Empty thewaste beaker into the sink at the end of the experiment.
Fill your burette with the prepared HCl solution,making sure that there are no air bubbles in the stopcock tip.(Make sure that the stopcock is closed first!)
With a 25.00 mL pipet and a bulb,pipet the prepared NaOH solution to a conical beaker.Add 2 or 3 drops of methyl orange indicator to the conical beaker.
Record the initial burette reading on the data sheet provided.All volume recordings should be to±0.01 mL!Add HCl drop by drop while swirling the conical beaker at the same time.When the color changes from yellow to red,you have reached the titration endpoint.Record the final burette reading(±0.01 mL).
Clean the burette thoroughly,and then rinse your burette with a small amount of the prepared NaOH solution (～5 mL)three times.Fill your burette with the prepared NaOH solution,making sure that there are no air bubbles in the stopcock tip.
Pipet 25.00 mL NaOH solution to a conical beaker.Add 2 or 3 drops of phenolphthalein indicator to the beaker.
Record the initial burette reading on the data sheet provided.Add NaOH drop by drop while swirling the conical beaker at the same time.When the faint pink persists for 30 sencond,you have reached the titration endpoint.you have reached the titration endpoint.Record the final burette reading.All volume recordings should be to±0.01 mL!
1.Can we prepare HCl or NaOH standard solution with accurate concentration directly?
2.What is meant by accuracy and precision in a scientific measurement?
3.In this experiment,is the volume ratio of titration NaOH with HCl and titration HCl with NaOH the same?