Idea Transcript
Calibration of a Pipet and Acid Titration Today you are given a chance to brush up on some of the techniques that you will be using during the remainder of the semester. Lab grades will be based on obtaining the correct answer in your analyses. Proper weighing and volumetric techniques are essential for successful completion of the classical methods labs. This lab is designed to help you become thoroughly familiar with the balances, flasks, pipettes, etc.
A.
Calibration of Glassware
Calibration of a pipet will allow you to develop your technique. Your results are easily checked and a mistake is not as costly as it may be during an analysis. 1. Your instructor will direct you how to clean a 25 mL volumetric pipet. Rinse pipet thoroughly with deionized (DI) water after cleaning. 2. Clean off and dry the outside of a 150 mL Erlenmeyer flask. Dry the inside of the neck of the flask. Seal the flask with Parafilm. 3. Weigh the flask. You will need balance sensitivity of at least 1 milligram. 4. Carefully fill the pipet to the mark with distilled water after measuring the temperature of the water with a thermometer. Deliver the contents into the 150 mL flask taking care not to wet the neck of the flask. Observe all of the rules of pipetting (if you are not sure, ask). DO NOT force out the last drop, merely touch the tip of the pipet to the side of the flask. 5. Weigh the flask and the water. Obtain the weight of water by difference and calculate the volume of the pipet using the temperature corrected density of water. (See Harris, Table 2‐7, page 42 for the density of water. 6. Repeat the determination until you get three results that agree within 0.1 %. Calculate the mean and standard deviation of the three measurements and record them in your lab notebook. Divide the standard deviation by the mean and multiply by 100% to determine if your results agree within 0.1 %. Notify your TA if your pipet volume differs by more that 0.1 mL from the stated volume. This procedure should be followed when extremely precise results are required. It will not be necessary to repeat this calibration for most of the titrations that you will perform. Make sure you use your calibrated pipet the rest of the semester.
B.
Simple Titration ‐ Use of the Buret
Titrations are performed by delivering a variable volume of reagent to the unknown solution using a buret. A 50.0 mL buret will be available for titrations. Like the pipet it is a precision tool, and with proper technique, it can be used to deliver various amounts of liquids with a precision of a few parts per thousand (0.2 ‐ 0.3 %). After checking the buret for cleanliness (No droplets adhering to the inside of the glass after a distilled water rinse), rinse with two small volumes (~5 mL) of the solution to be used. Fill to near the zero mark Calibration of a Pipet and Acid Titration
and dispense some of the liquid through the stopcock to purge air bubbles from the tip. After removing the bubbles take an initial reading. You will subtract this initial volume from the final volume after the endpoint has been reached. Errors in both initial and final readings will contribute to the total uncertainty in the volume delivered. The buret should be kept clean when not in use. The tip should not be broken or chipped. If it is chipped, replace the tip. Your instructor will describe the correct way to clean your buret. The same general method used for calibration of your pipet may be used for your buret. It is generally safe to assume that the marked gradations on your buret are accurate but you may wish to check this by calibration when high precision is required.
Procedure
You will be given a standardized solution of NaOH (~0.1 M – check the bottle for the actual concentration). Use your calibrated pipet to add 25.0 mL of this solution into a clean 250 mL volumetric flask. Dilute the contents to the mark with distilled water and mix thoroughly. (What is the concentration of the solution in the flask?) You will use this solution in your buret to titrate an unknown solution of HCl. Be sure to clean your pipet by flushing it several times with DI water. If the pipet has only been in contact with ionic solutions, it is not necessary to clean it with soap.
Pipet 25.0 mL of the HCl unknown (“unknown” indicates that you do not know the concentration) into a clean 250 mL volumetric flask. Dilute to the mark and mix thoroughly. Pipet 25 mL of the diluted acid into a 250 mL Erlenmeyer flask, add three drops of phenolphthalein indicator and titrate to the endpoint (change from colorless to the first detectable pale pink). The color change should be noticeable over an addition of only 1 drop of titrant. This should be performed three times.
Perform blank titrations in exactly the same manner as was used to titrate your unknown. A blank titration includes everything but the actual analyte; therefore, your “sample” is 25 mL distilled water. The blank titration will tell you how much titrant is necessary for the indicator to change color (normally a few drops). Make sure you subtract the blank titration volume from the analyte titration volume for your unknowns. This is known as the titration error.
The relative uncertainty in the volume delivered depends on the actual volume delivered. If only 1.00 mL is delivered the uncertainty might be 1.00 ± 0.03 or 3 % relative error but if 50.00 mL are delivered the relative error will only be (0.03/50.00) /100 = 0.06 %. For this reason it is desirable to adjust the concentration of the titrant so that about 20 mL are used in a titration.
Report the concentration of HCl unknown (i.e., the original 25 mL) in moles/liter from the known stoichiometry of the acid‐base neutralization reaction, the known concentration of the NaOH titrant, and the known dilutions of the HCl solution. Values are to be reported on the Experiment Report Sheets.
The lab report for Experiment 1 should include the volume of your 25 mL pipet along with its associated error and the concentration of the HCl solution with its associated error. (See Harris, Chapter 2, for common glassware tolerances, errors.) See Appendix 2 for an example of error analysis calculations.
Calibration of a Pipet and Acid Titration
Student Name:
Quant Student
Chemistry 3200 Calibration of a Pipet and Acid Titration Date:
Lab Instructor:
Quant TA
Unknown Number:
Section:
A
1
Part A: Calibration of 25 mL pipet 22° C Temperature of water: Correction factor for volumetric calibration at aforementioned temperature:
1.0033 ml/g Grams of water:
24.9036 g 24.9020 g 24.9028 g Volume of water (from temperature corrected density of water):
24.9860 mL Volume of 25 mL pipet:
24.9842 mL 24.9850
24.9850 mL 0.0008 mL
Part B: HCl Titration Concentration of standardized NaOH stock solution: 0.09882 0.00005 M Concentration of titrant solution:
0.009882 0.000007
M
mL NaOH added (blank corrected):
25.60 mL
25.70 mL
mL NaOH (average + standard deviation):
25.80 mL 25.7 0.1
mL 0.000254 0.000006 moles Molarity of original HCl sample: 0.1017 0.0004 M Moles HCl in titrated sample:
Calibration of a Pipet and Acid Titration
Student Name:
Quant Student
Calculation for volume of 25 mL pipet: V3 = massH2O × (Volume of 1g of H2O in ml) Table 2‐7 page 42 V3 = 24.9028 g × 1.0033 mL/g = 24.9850 mL V1 = 24.9860 mL V2 = 24.9842 mL VAve = 1/3 ×(V1 + V2 +V3) = 24.9850 0.0008 mL standard deviation of 3 measurements (Standard deviations are most easily calculated on your calculator or in Excel – either is fine.) Calculation for HCl molarity: (Take #1 as the Example) mL NaOH: (Ave Standard Deviation) = 1/3 × (25.70 + 25.80 + 25.60) = 25.80 0.1 mL
Concentration of NaOH titrant: MdilVdil = MconcVconc Mdil = 0.09888×24.9850/250.00 = 0.00988207 Keep the largest number of significant figures Mdil = 0.009882 7×10‐6 Considering the error analysis below
Moles HCl in titrated sample: (0.02570 L NaOH) × (0.009882 mol NaOH/L) × (1 mol HCl/1 mol NaOH) = 0.0002540 mol HCl molHCl = 0.000254 1×10‐6 Considering the error analysis below
Concentration of HCl in titrated sample: 0.000254 mol HCl / 0.0249850 L = 0.01016610 M HCl
Molarity of Original HCl sample: M1V1 = M2V2 M2 = M1 × (V1 / V2 ) = 0.01016610 M × (250 mL / 24.9850 mL) = 0.1017220 M HCl M2= 0.1017 0.0004
Calibration of a Pipet and Acid Titration
Student Name:
Quant Student
Calculation for error analysis (Include a list the errors and their sources. You may neglect errors that are negligible WITH justification.): (Take #1 as the Example) Sources of error Relative error in pipet = 0.0008/24.9850 = 0.00003 Keep one significant digit for errors Relative error in volumetric flask = 0.12/250.00 = 0.0005 Chapter 2 Relative error in molarity of NaOH = 0.00005/0.09888 = 0.0005 Relative error in buret = 0.05/25.8 = 0.002 Chapter 2 Relative error in titration = 0.1/25.8 = 0.004 Uncertainty in diluted NaOH Errors involved are pipette, volumetric flask and molarity of NaOH errors. The pipet can be neglected because it is one order of magnitude smaller. Relative uncertainty in Mdil = (0.00052 + 0.00052)0.5 = 0.0007 Absolute uncertainty in Mdil = 0.00988207 × 0.0007 = 7×10‐6 Uncertainty for moles HCl in titrated sample: Errors involved are the buret, titration and molarity of NaOH errors. The molarity error can be neglected due to magnitude. Relative uncertainty in molHCl [ (0.002)2 + (0.004)2]0.5 = 0.004 Absolute uncertainty in molHCl = 0.004 × 0.0002540 = 1×10‐6 Molarity of original HCl sample: Errors involved are errors in molHCl , pipette, volumetric flask and pipette again. The error in the volumetric flask and pipette can be ignored due to magnitude. Relative error in M2 = 0.004 Absolute error in M2 = 0.004 × 0.1017220 = 0.0004
Calibration of a Pipet and Acid Titration
Student Name:
Chemistry 3200 Calibration of a Pipet and Acid Titration Date:
Lab Instructor:
Section:
Correction factor for volumetric calibration at aforementioned temperature:
Unknown Number:
Part A: Calibration of 25 mL pipet Temperature of water:
Grams of water:
Volume of water (from temperature corrected density of water):
Volume of 25 mL pipet:
Part B: HCl Titration Concentration of standardized NaOH stock solution:
M
Concentration of titrant solution:
M
mL NaOH added (blank corrected):
mL NaOH (average + standard deviation): Moles HCl in titrated sample:
Molarity of original HCl sample:
Calibration of a Pipet and Acid Titration
Student Name:
Calculation for volume of 25 mL pipet: Calculation for HCl molarity: Calculation for error analysis (Include a list of the errors and their sources). You may neglect errors that are negligible WITH justification.)
Calibration of a Pipet and Acid Titration