Read Technique 9(including Figure 0-11) in this manual for instructions on the u
ID: 1060775 • Letter: R
Question
Read Technique 9(including Figure 0-11) in this manual for instructions on the use of a pipet. Read the section on Beer's Law in Experiment 10 and the directions for the Spectronic 20 in the Appendix for information on how to perform the spectrophotometric analysis. Preparation of Standard Nickel Solutions: In this part of the procedure, you will create a set of three solutions of known concentration in NiCl_2-6 H_2O and measure their absorbance. You will use this data to graphically determine the concentration of nickel in the unknown solution. See the introduction to this experiment for further explanation. Obtain a clean, rinsed 50 mL volumetric flask and a clean, dry 150 mL beaker. Weight the beaker on the analytical balance and record the mass to 0.001 g on the data sheet. Add approximately 1.1 g of NiCl_2-6 H_2O, and record the mass of the n=beaker abd its contents exactly. Dissolve the crystals completely in about 10 mL of 0.5 M H_2SO_4, with stirring. Transfer the solution to the 50 mL volumetric flask using your stirring rod to avoid losing any solution. Wash the beaker three times with 3 mL portions of 0.5 M H_2SO_4, transferring the washings quantitatively to the flask. Dilute the solution in the flask exactly to the mark with 0.5 M H_2SO_4, stopper, and mix thoroughly by inverting the flask several times. See "Technique 4: Preparing Solutions" in the introduction to this manual for an explanation of the use of a volumetric flask. Calculate [Ni^2+] of nickel in this stock solution. The stock solution will serve as one standard solution; you will need to create two others, so that you can measure how absorption changes as a function of concentration. Prepare the two additional standards diluted as follows: Pipet 10.0 mL of the stock solution into a clean, rinsed 25 mL volumetric flask and dilute to the mark with 0.5 M H_2SO_4(invert twice to mix thoroughly). Then pipet 20.0 mL of stock solution into a second 25 mL volumetric flask and dilute with acid (mix). Calculate [Ni^2+] in each of these dilute standard solutions. Preparation of the Complex Compound Solution: Place approximately 0.6 g of your complex compound m a pre-weighed 150 mL beaker, and record the mass of your complex as exactly as possible. Dissolve in roughly 30 mL of H_2SO_4 and transfer to a 50 mL volumetric flask. Rinse with H_2SO_4 and dilute the solution to exactly 50 mL, as described above for the preparation of the stock solution. Spectrophotometric Measurements: Obtain two cuvettes (see directions for how to handle these). One will be a blank (see instructions on the use of the Spectronic 20), while the other will contain the solution to be measured. Fill the blank cuvette to its fill line with 0.5 M H_2SO_4. Fill the other cuvette to the fill line with one of the stock solutions; you should use the same blank in all of your measurements, so do not discard this solution until the experiment is complete. Make a measurement with the Spectronic 20 (the wavelength should be 395 nm) and record the absorbance. Empty the stock solution into waste container, then rinse the cuvette with distilled water and dry it. Then measure the absorbance with each of the other two standard solutions. Finally, measure the absorbance of the solution containing your synthesized compound.Explanation / Answer
Ans.
A. Preparation of standard solution.
1. Accurately weigh 1.1 gram NiCl2.6H2O and carefully transfer it to a 50 mL standard volumetric flask. Dissolve it in little amount of 0.5 mH2SO4. Make the solution upto the mark with 0.5M h2SO4 solution.
Moles of NiCl2.6H2O = mass / molar mass
= 1.1 g / 237.68 g mol-1 = 0.0046280 moles
Since, 1 mol NiCl2.6H2O has 1 mol Ni, hence, number of moles of Ni is also equal to 0.0046280.
Concentration of Ni = moles/ volume of solution in L
= 0.0046280 moles / 0.050 L [50 mL = 0.050 L]
= 0.09256 M
This solution serves as standard stock solution.
B. Preparation of standard aliquots.
2. Aliquot 1: Transfer 10.0 mL of standard solution to 25 mL standard volumetric flask. Make the final volume upto the mark with 0.5M H2SO4.
Using, M1V1 = M2V2 , where
M1= molarity of initial solution 1, V1= volume of initial solution 1 ;(standard stock solution)
M2= molarity of final solution 2, V2= volume of final solution 2 ;(aliquot 1)
0.09256 M x 10 mL = M2 x 25 mL
Hence, M2 = 0.037024
Thus, [Ni2+] in aliquot 1 = 0.037024 M
3. Aliquot 2: Transfer 20.0 mL of standard solution to 25 mL standard volumetric flask. Make the final volume upto the mark with 0.5M H2SO4.
Using, M1V1 = M2V2
0.09256 M x 20 mL = M2 x 25 mL
M2 = 0.074048
Thus, [Ni2+] in aliquot 2 = 0.074048 M
C. Preparation of unknow solution
Prepare unknown solution as described in part A (preparation of standard solution) with 0.6 g unknown sample.
D. Measuring absorbance
4. Take around 1.0 ml sample from standard stock solution into 1 mL quartz cuvette.
5. Take the absorbance of this solution at 395 nm against reagent blank (0.5 M H2SO4).
6. Similarly take the absorbance of aliquots 1 and 2 against reagent blank
7. Take the OD of the unknown solution against reagent blank.
Note: take at least 3 absorbance readings for for each solution. Use average absorbance to plot the graph.
D. Preparation of standard graph and extra-plotting the unknown
Prepare the standard graph by taking the absorbance of standard solutions (stock, aliquot 1 and aliquot 2) on Y-axis and their respective concentrations on X-axis.
Use excel sheet of similar software to generate the linear graph equation.
Mark the absorbance of unknown solution on Y-axis. Extra-plot it to X-axis to know the concentration. Or, use the graph equation to calculate the concentration of the unknown.
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