Chem 231-062 Lab Final
What are the units for molar absorptivity? (Exp #5 UV-VIS)
(Molarity-1)(cm-1)
Exp 5: UV-VIS
-Determinant, Solver, and Difference methods -Solver is most accurate -Difference is most selective
Exp 2: Chloride Notes
-Fajan's method is used -Ag+ is the titrant -before eq point, excess of Cl- -adsorption indicator, dichlorofluorescein -signals endpoint by reacting on the surface of the precipitate formed in the titration, silver chloride -produces a precipitate/indicator complex which shows its characteristic color -Mohr and Volhard methods (Mohr - formation of a precipitate, chromate indicator) (Volhard - formation of a colored complex, Fe3+ is added)
Exp 1: Glassware Notes
-TC= to contain -TD= to deliver -transfer pipets deliver a fixed volume of liquid -graduated pipets deliver volume given by graduations -micropipettes can deliver volumes of 0..2-1000 microliters, are adjustable (amount of liquid delivered is dependent on the volume selected) and fixed -burets deliver a certain volume of liquid, usually in titrations, with a controlled release due to the stopcock -volumetric flasks contain a specific volume of liquid usually in order to make standard solutions and dilute conc solutions -concluded transfer pipet was most accurate
Exp 3A: Acetic Acid
-compares 2 diff experimental methods of finding the endpoint of an acid-base titration, acid-base indicator vs pH meter -titrate a weak acid (HA) with a strong base (OH-) -in beginning weak acid is the only component in water -using an acid-base indicator, HInd, weak acid indicator -acid and conj base have diff colors -as titrant is added, the pH increases and the reactionshifts to the right so the color A begins to decrease and color B begins to increase -In this experiment, 3 different indicators are considered: Methyl red (pKInd=4.95), Bromothymolblue (pKInd=7.1), and Phenolphthalein (pKInd=9.3). The indicators will not change color at aspecific pH but rather over a narrow range of pH as described in the table below -The indicators color range will be within ± 1 of the equivalence pH. -Using a pH meter, a titration curve can be plotted. -A steep inflection will be seen; the pH at the equivalence point for thetitration of a weak acid with a strong base will be above 7.
Exp 3: KHP Notes
-setting up a neutralization reaction which involves neutralizing the hydrogen phthalate anion and getting the divalent phthalate anion as the product -acid-base titration -strong base titrant -approx conc of NaOH solution can be standardized using pure KHP, primary standard acid -equivalence point on basic side -phenolphthalein indicator
Exp 7: HPLC
-stationary phase- particles packed into a column -mobile phase- pumped through the column, consists of strong and weak solvent -sample injected into column, interacts with both mobile and stationary phases, sample separated based on the time each component spends interacting with each phase -retention time is the time the compound spends in the column -under high amounts of strong solvent, compounds will see a decrease in the interaction with the stationary phase, and thus a shorter retention time -isocratic separations- mobile phase is kept the same throughout the run -reversed phase with 2 components, weak solvent=water, strong solvent=organic solvent -broadening effect on later peaks -gradient separation- mobile phase is changing over the course of the run, moving to higher amounts of strong solvent -for reversed phase, the percent of organic solvent is increased over the run -gradient is optimal if there is a large difference in the retention times of the first and last compound u
Exp 4: Water Hardness
-uses an ammonia-ammonium chloride buffer -EDTA is added as the free metal cation is complexed and then when used up, the metal is displaced from the indicator, EBT, and complexed to EDTA causing the color change from red to blue. -complexing agent EDTA, chelating agent -ammonia attaches itself to AgCl and converts it to its water soluble form -In this experiment, EDTA complexes with the ions that contribute to water hardness in a1:1 stoichiometry. The indicator used in this reaction is EBT which is also a chelating agent and has 3 ionizable protons. The color complexed with Mg2+ in solution is red. As EDTA is added, the free metal cation is complexed first due to the higher stability of the Mg-EDTA complex compared to MgIn-. When the free metal cation is used up, the metal is displaced from the indicator and complexed to EDTA, which causes the color change from red to blue. -small amount of Mg2+ is added as Mg-EDTA to get a sharper endpoint.
Exp #5 UV-VIS Pre-Lab Question: 2.0 mL of a 0.750 M solution is diluted to 80.0 mL. Then, 40.0 mL of this new solution is diluted to 250 mL. Finally, 25.0 mL of this solution is diluted to 175 mL. What is the concentration in moles per liter of the final solution?
0.00427 M First dilution: m1v1=m2v2, solve for m2 Second dilution: m1v1=m2v2, use molarity from first as m1, solve for m2 Third dilution: m1v1=m2v2, use molarity from second as m1, solve for m2
Exp #3 KHP Pre-Lab Question: Calculate the molarity of a 50.5% by weight NaOH solution having a density of 1.53 g/mL.
19.3 M NaOH g solution/mL to g NaOH to mol NaOH, convert to L
What the equivalence of 0.10mol/L H3PO4 (sample, pKa1=2.14, pKa2=7.20, pKa3=12.37) in titration VS 0.10mol/L NaOH (titrant) with thymolphthalein (pKIn=9.9) as the indicator? (Exp #3 KHP)
2
Exp #2 Chloride Pre-Lab Question: A 0.4571 g sample of a chloride containing unknown was titrated with 0.1013 M AgNO3 using the Fajans' method, requiring 42.33 mL to reach the faint pink endpoint. Calculate the percent chloride in the sample.
33.25% (L AgNO3 to mol AgNO3 to mol Ag+ to mol Cl- to g Cl-) % chloride = g Cl-/g sample *100
Which of the following buret readings contains the correct number of significant digits? (Exp #2 Chloride)
35.47
Exp #3A Acetic Acid Pre-Lab Question: Determine the equivalence volume of a titration of 10mL of a 0.38M acetic acid solution with 0.1 M NaOH?
38 mL or 0.038 L m1v1=m2v2
Exp #3 KHP Pre-Lab Question: How many mL of a .1000 M NaOH solution are required to titrate a solution containing 0.8000 g of pure potassium acid phthalate (KHP)?
39.17 mL g KHP to mol KHP to mol NaOH, use original molarity and mol found to get L solution, convert to mL
Exp #4 Water Hardness Pre-Lab Question: Report the water hardness as ppm CaCO3 of a 50.00 mL water sample that required 32.50 mL of 0.0100M EDTA to reach the end point of the titration.
650 ppm CaCO3 find mol EDTA from volume and molarity, mol EDTA=mol CaCO3, find g CaCO3, convert to mg, divide by L solution
Exp #3A Acetic Acid Pre-Lab Question: Calculate the pH at the equivalence point.
8.82 find Kb from Ka, find conc OH-, then pOH, finally pH
Exp #4 Water Hardness Post-Lab Question: Using the calcium ion as a typical representative, just how high a pH do you think could be considered as "too high" for a solution with a hardness of about 250 ppm CaCO3? Ksp for Ca(OH)2 is 6.5x10^-6
A pH higher than 12.71 would be considered too high. mg/L CaCO3 to mol/L Ca2+, set up ksp expression, solve for x, 2x=conc OH-, find pOH, then pH
What pushes the mobile phase through an HPLC instrument? (Exp #7 HPLC)
A pump
What kind of indicator are we using (by what process does it react)? (Exp #2 Chloride)
Adsorption
What is the important distinction for acid-base indicators? (Exp #3A Acetic Acid)
An important distinction for acid-base indicators is the pH range at which they change color. Another important distinction is the transition of colors. For example, phenolphthalein changes color at a range of 8.3-10.3 from clear to faint pink. These characteristics are specific to the indicator used.
Exp #5 UV-VIS Post-Lab Question: How does the experimentally determined accuracy of each spectroscopic method (Determinant, Solver, and Difference) compare with what you predicted from theoretical considerations? If the accuracy ranking you observed for the test mixture (e.g., Determinant > Solver > Difference) is not the same as what you predicted theoretically, please explain how this could be so (sources of error, etc.).
Based on the experimentally determined accuracy of each method, the accuracy ranking above is justified. The highest relative error, which was 2.59 or 259%, resulted from determining the concentration of in the test mixture using the Difference Method. The lowest relative error, which was 0.80 or 80%, resulted from determining the concentration of in the test mixture using the Solver Method. Based on these results, the Solver Method is the most accurate and the Difference Method is the least accurate which matches with the theoretical considerations above. The order of accuracy from least to greatest is: Difference < Determinant < Solver
Exp #5 UV-VIS Post-Lab Question: Based on theoretical considerations, which of the spectroscopic methods (Determinant, Solver, and Difference) should be the most accurate for a two-component mixture, assuming that all three methods are applicable based on the individual spectra of these components? Which is the least accurate? Please explain your answer for both questions.
Based on theoretical considerations, the Solver Method should be the most accurate for a two-component mixture. This is because the Solver Method considers data from 5 different wavelengths instead of just 2 for both the Determinant and Difference Methods. The Solver Method is good to use when the absorbance spectra overlap significantly and/or when the absorbance and molar absorptivity measurements are imprecise. Having a larger dataset increases the accuracy of the results when determining the concentrations. The least accurate method is the Difference Method. This method has the most specific requirements and therefore is not accurate for all instances. It is only possible when the absorption spectrum of the mixture over some portion of the wavelength range is due solely to the absorbance of only one of the analytes. The absorbance and molar absorptivity of one of the analytes is zero over that portion of the wavelength range. Assuming that the absorbance and molar absorptivity is z
Which of the following standard solution can be prepared by weighing? (Exp #3 KHP)
Borax (sodium tetraborate decahydrate)
Which indicators have two color changes? (Exp #3A Acetic Acid)
Bromothymol blue and bromocresol green
Exp #4 Water Hardness Pre-Lab Question: Write the equation for the titration of Ca2+ with EDTA.
Ca2+ +EDTA4- --> Ca(EDTA) 2-
Calcium bicarbonate + calcium hydroxide = ????? + ?????? (Exp #4 Water Hardness)
Calcium carbonate and water
What is the name of the object which holds the sample in the UV-Vis spectrometer? (Exp #5 UV-VIS)
Cuvette
What is the name of the indicator used in this lab? (Exp #2 Chloride)
Dichlorofluoroscein
What are the 3 methods used to determine the concentration of an unknown mixture? (Exp #5 UV-VIS)
Difference, Determinant, Solver
Which of the following is not a reagent you will be using in this lab? (Exp #2 Chloride)
EBT
What is the name of the point in a titration where the indicator changes color? (Exp #2 Chloride)
End point
A gradient mobile phase is kept the same throughout the run. (Exp #7 HPLC)
False
Adding more indicator will give you better titration result. (Exp #3 KHP)
False
Bubbles in the burette will not affect the result. (Exp #3 KHP)
False
For any given compound (given the same detector parameters) the concentration of an unknown sample of a known compound can be determined so long as you know the retention time and the area. (Exp #7 HPLC)
False
The indicator phenolphthalein turns a solution from clear to pink as the pH decreases. (Exp #3A Acetic Acid)
False
Ca2+ is added as Ca-EDTA for a sharper endpoint. (Exp #4 Water Hardness)
False, a small amount of Mg as Mg-EDTA is added
True or False. Phenolphthalein is the indicator used in this experiment. (Exp #2 Chloride)
False, dichlorofluorescein is the indicator
The analyte is put in the buret during a titration. (Exp #3A Acetic Acid)
False, the titrant is
_____ are based on the mass of the final product. (Exp #2 Chloride)
Gravimetric methods
Exp #3A Acetic Acid Post-Lab Question: Using the information provided about the 2 unused indicators, describe what observations you would have seen if you had used them instead for your titrations?
If the other 2 unused indicators, Methyl Red or Bromothymol Blue were used instead for the titrations, the pH at the equivalence point would not be accurate. The indicator should have a color change pH range within +/- 1 of the equivalence pH. The equivalence pH for this titration was around 9. Phenolphthalein was the best indicator because it was able to change color around this pH to accurately indicate when the equivalence point was near. If Methyl Red was used instead, which has a pH range of 3.95-5.95, it will not be able to indicate the pH at the equivalence point and therefore can lead to relatively large errors. On the curve, this pH is closer to the midpoint rather than the equivalence point. If Bromothymol Blue was used instead, which has a pH range of 6.1-8.1, it will not be able to indicate the pH at the equivalence point and therefore also lead to relatively large errors. On the curve, this pH range is slightly lower than the pH at the equivalence point.
Exp #5 UV-VIS Pre-Lab Question: In all of the calculations involving concentration in this experiment, we use the spectroscopic unit absorbance, yet during the experiment it is better to record the percent transmittance when using the Spectronic 20. Why?
It is better to record the percent transmittance when using the Spectronic 20 because the %T scale is linear while the absorbance scale is logarithmic. It is easier and more accurate to interpolate a linear scale than a logarithmic one so that is why the %T readings are then used to convert to absorbance.
What are the three choices for indicator in this lab? (Exp #3A Acetic Acid)
Methyl red, bromothymol blue, and phenolphthalein
Exp #4 Water Hardness Pre-Lab Question: Write the equations for the endpoint color change of Eriochrome Black T (EBT) with Mg2+.
Mg2+ + HIn2- --> MgIn- + H+ MgIn- (red) + H+ + EDTA4- --> Mg(EDTA)2- + HIn2- (blue)
When reading a UV-Vis calibration curve, what does the slope represent? (Exp #5 UV-VIS)
Molar absorptivity
What type of titrant are we using (by what process does it react)? (Exp #4 Water Hardness)
Na2EDTA is the titrant used in this experiment; it is a ligand and is involved in complexation reactions.
The strong base used in this experiment is... (Exp #3A Acetic Acid)
NaOH
Which of the titrations has an equivalent point with pH=7? (Exp #3 KHP)
NaOH VS HCl
What will the color of your solution be once the indicator is added? (Exp #2 Chloride)
Neon green
At the equivalence point, which ion is in excess? (Exp #2 Chloride)
None
Which is a requirement for the use of the difference method? (Exp #5 UV-VIS)
One compound has no absorbance at one of the chosen wavelengths
Exp #4 Water Hardness Post-Lab Question: When Mg-EDTA is added to a sample to compensate for lack of , why is no blank correction necessary?
Only a very small amount of is added as Mg-EDTA for a sharper endpoint; it is such a small amount that it does not have a significant effect on the final result. There is no effect of the blank on the standards; therefore, it is unnecessary to do a blank.
What is a proper container to store NaOH solution (for acid-base titration)? (Exp #3 KHP)
Polyethene narrow mouth bottle with threaded neck and Polypropene stopper (has to be something plastic, not glass)
In the video, what is the last step of the titration? (Exp #3A Acetic Acid)
Read and record the volume of titrant (Sodium Hydroxide) used
What are two main types of HPLC modes? (Exp #7 HPLC)
Reversed-Phase and Normal Phase
Briefly describe how scum forms. (Exp #4 Water Hardness)
Scum forms because Calcium and Magnesium ions present in hard water react with the salts of higher fatty acids producing calcium and magnesium salts which are not soluble in water and remain on the surface of water as precipitates or scum causing little lather.
Describe the ion-exchange method to remove hard water ions from a sample. (Exp #4 Water Hardness)
Small balls of resins are packed in tubes. The hard water essentially passes through the tube and sodium ions from the resin come off and go into the water. They exchange with the calcium ions therefore removing hard water ions from the sample.
Which method theoretically should give the most accurate results for a mixture? (Exp #5 UV-VIS)
Solver method
Describe Clark's method to remove hard water ions. (Exp #4 Water Hardness)
The Clark method is when lime, or calcium hydroxide, is added to hard water. The calcium hydroxide reacts with calcium bicarbonate to form calcium carbonate and water. At the same time, magnesium bicarbonate reacts with calcium hydroxide to form calcium carbonate, magnesium hydroxide, and water. The calcium carbonate and magnesium hydroxide do not dissolve and form precipitates which can be removed through filtration.
What is the endpoint color of this titration? (Exp #4 Water Hardness)
The endpoint color of this titration is blue.
Exp #3 KHP Post-Lab Question: Explain the difference between the endpoint and the equivalence point.
The equivalence point in an acid-base titration indicates the completion of the reaction. At this point, the number of moles of titrant and the analyte are equal. For example, in this experiment, at the equivalence point, the number of moles of KHP is equal to the number of moles of NaOH. The endpoint comes after the equivalence point. It indicates that the equivalence point has been reached; it is accompanied by the change in color of the solution when the indicator changes color.
Exp #3 KHP Post-Lab Question: Phenolphthalein indicator is used in this experiment because it changes color in the pH range of 8.0 to 9.6. Show by calculation that phenolphthalein is an acceptable indicator for the titration of KHP with NaOH.
The pH at the equivalence point is 9.04 which is within the pH range of the indicator, which is 8.0-9.6 so phenolphthalein is an acceptable indicator for titrating KHP.
Permanently hard water is caused by what types of chemical compounds? (Exp #4 Water Hardness)
This is due to the presence of chloride or sulfate salts of calcium or magnesium.
Why is it important to dilute samples before reading them on a UV-Vis? (Exp #5 UV-VIS)
To make sure that they are within the readable range of the instrument
What is the purpose of using a 'blank'? (Exp #5 UV-VIS)
To show the background spectra, when there is no sample my answer: By using a blank, different types of contamination can be identified.
What is the correct behavior when you see some solution hanging on the tip during titration? (Exp #3 KHP)
Touch the tip with Erlenmeyer flask inside wall and use wash bottle to flush the Erlenmeyer flask with deionized water
A good separation has well resolved peaks. (Exp #7 HPLC)
True
According to the equation for Beer's Law, Abs and Conc have a linear relationship. (Exp #5 UV-VIS)
True
At the beginning of a titration of a weak acid and a strong base there is only the weak acid in water. (Exp #3A Acetic Acid)
True
EBT or calmagite cannot be used to indicate the titration of Ca2+ alone because it forms too weak a complex with the indicator and therefore does not give a sharp endpoint. (Exp #4 Water Hardness)
True
EDTA complexes with the ions contributing to water hardness in a one-to-one stoichiometry. (Exp #4 Water Hardness)
True
Except at very high concentrations, Beer's Law is additive at all wavelengths. (Exp #5 UV-VIS)
True
One of the dectors used on an HPLC instrument is a UV-Vis Detector. (Exp #7 HPLC)
True
Sodium Hydroxide is the titrant in the titration video. (Exp #3A Acetic Acid)
True
The burette should be rinsed with titrant before starting the titration. (Exp #3 KHP)
True
The end point and equivalent point are close in a good titration. (Exp #3 KHP)
True
True or False. A vortex will ensure adequate mixing of the solution during the titration. (Exp #2 Chloride)
True
You can titrate 0.10mol/L CH3COOH solution (sample) with 0.10mol/L NaOH solution (titrant), but you cannot titrate 0.0010mol/L CH3COOH solution (sample) with 0.0010mol/L NaOH solution (titrant). (Exp #3 KHP)
True
Exp #2 Chloride Post-Lab Question: In a volumetric determination using a 50 mL buret, it is advisable to use at least 30 mL of titrant if possible. Why do you think this is so?
When using a 50 mL buret, it is advisable to use at least 30 mL of titrant if possible so that there is enough titrant added to reach the endpoint. If there is not enough titrant in the buret before starting the titration, more will have to be added in later on increasing the chances of error. Having enough titrant from the beginning ensures there is a continuous titration which results in more accurate and precise results.
When does the detector measure the analytes? (Exp #7 HPLC)
after its separation in the column
How is retention time calculated? (Exp #7 HPLC)
sum of the time spent in the mobile phase and time spent in the stationary phase
What is used to remove dissolved gasses from the solvents? (Exp #7 HPLC)
the pump and degasser
What is retention time? (Exp #7 HPLC)
the time the compound spends in the column
What color change is observed when the indicator methyl red is used? (Exp #3A Acetic Acid)
yellow to red
