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SOLUTION CONCENTRATIONS
Introduction

To learn how to prepare standard molar solutions and to become acquainted with the principles of colorimetric analysis.

The Assignment

1.Solution Preparation 1.For this experiment, you must calculate the mass of solute, nickel (II) nitrate, needed to prepare the four following standard solutions: 1.0.050 M Ni(NO3) 2in a 100 mL volumetric flask.
2.0.100 M Ni(NO3) 2in a 100 mL volumetric flask.
3.0.150 M Ni(NO3) 2in a 100 mL volumetric flask.
4.0.200 M Ni(NO3) 2in a 100 mL volumetric flask.

2.Prepare an Excel® spreadsheet with the following headings: 1.Solution Concentration
2.Mass of Solute Needed
3.Absorption at 400 nm, trial number 1
4.Absorption at 400 nm, trial number 2
5.Absorption at 400 nm, trial number 3
6.Average Absorption at 400 nm

3.Enter the data for Solution Concentration and Mass of Solute for the blank and each of the four nickel (II) nitrate solutions.

2.Preparation of the Calibration Curve 1.A technician has run three trials and has obtained absorbance readings for each of the solutions.
2.The technician’s results are as follows: 1.For the blank, 0.000 M Ni(NO3) 2, the results for Absorbance are: 0.00, 0.00 and 0.00.
2.For the 0.050 M Ni(NO3) 2 solution, the results for Absorbance are: 0.23, 0.24, and 0.26.
3.For the 0.100 M Ni(NO3) 2 solution, the results for Absorbance are: 0.50, 0.49, and 0.49.
4.For the 0.150 M Ni(NO3) 2 solution, the results for Absorbance are: 0.75, 0.75, and 0.74.
5.For the 0.200 M Ni(NO3) 2 solution, the results for Absorbance are: 1.03, 0.99, and 1.00.

3.Record each of these values in the appropriate cell of your Excel® file.
4.Calculate the average Absorbance, record them in the appropriate cells of your Excel® file, and use these values for the Absorbance points on your graph.
5.Use the chart function of Excel® to plot an XY (scatter) diagram comparing Absorbance to concentration (molarity).
6.Plot your results with the independent variable, molarity of the nickel solution, along the abscissa, or x-axis, and with the dependent variable, Absorbance, along the ordinate, or y-axis.Move the columns of you Excel® chart as needed to obtain the correct plot; you may use additional “sheets” if you wish to accomplish this.
7.Print a copy of the graph.
8.On your print copy, use a straightedge to draw the best straight line through the points.Do not “connect the dots.”The line does not need to touch each point; it should be the most representative straight line so that there will be about the same number of dots above as below the line and at reasonably the same distance from the line.

3.Determination of Nickel 1.The technician, using a nickel (II) nitrate, Ni(NO3)2, solution of unknown concentration, has run three trials and has obtained the following Absorbance readings:0.42, 0.38, and 0.41.
2.Record these results in your Excel® file with the average Absorbance of these results.
3.Calculate the average absorbance and use this value for an absorbance point on your graph. Use the printed graph from above, find the average Absorbance of the unknown on the y-axis, and place a point on the straight line at that Absorbance.
4.Using the observed absorbance and your calibration curve, determine the molarity, or moles per liter, of the nickel (II) nitrate, Ni(NO3) 2, unknown solution. Again use the printed graph; find the point on the x-axis directly below the point on the straight line that represents the Absorbance of the unknown. This is the molarity of the unknown nickel (II) nitrate solution.
5.Enter this information into your Excel® file.
6.Calculate the mass of nickel (II) nitrate that would be found in 100 mL of the unknown solution and enter it into your Excel® file.
7.Plot a second graph with the independent variable, molarity of the nickel solution, along the abscissa, or x-axis, and with the dependent variable, Absorbance, along the ordinate, or y-axis.This graph will include the point for the unknown nickel (II) nitrate solution.
8.Review your graph to ensure that you have included the appropriate labels:Chart Title, the value that the x-axis represents, and the value that the y-axis represents.
9.Submit the Excel® file electronically to your instructor.


Resources


Chemists and laboratory technicians use a process called colorimetric analysis to meaure the variation in the intensity of the color of a solution with changes in concentration.The color may be due to an inherent property of the compound being evaluated or it may be developed in a reaction that forms a colored compound.By comparing the intensity of the color of a solution of unknown concentration with the intensities of solutions of known concentrations, the concentration of the unknown solution may be determined.In this experiment, the nickel (II) nitrate, Ni(NO3) 2, has an inherent color, green. The human eye can discern differences in color intensity with reasonable accuracy.However, in the laboratory we normally use an instrument known as a spectrophotometer or colorimeter in order to eliminate any variation in human judgment. The spectrophotometer works by comparing the amount of light at a specific wavelength that enters the sample with the amount that is transmitted through the sample.The amount of light absorbed is related to the concentration of the compound in solution.


The spectrophotometer has five basic components:
1.A light source that produces light in a wavelength range from about 375 to 650 nm.
2.A monochromator, which selects a wavelength of light specific to the sample being analyzed.
3.The sample cell, which contains the solution being analyzed.
4.A detector that measures the intensity of the light being transmitted.
5.A meter or display that indicates the intensity of the transmitted light.
For any compound in solution, the amount of light absorbed depends upon: 1.The concentration,
2.The cell or path length;
3.The wavelength of light; and
4.The solvent.

There are two common ways of expressing the amount of light absorbed.One is in terms of percent transmittance, T.Percent transmittance corresponds to the percentage of light transmitted, which is mathematically defined as the amount of light leaving the sample divided by the amount of light entering the sample times 100%.The amount of light entering the sample is found by measuring the intensity of light that is transmitted through a blank sample, i.e. through a cell that contains only the solvent.


Another method of expressing the amount of light absorbed is in terms of Absorbance, A, which is mathematically defined as the logarithm of the amount of light leaving the sample divided by the amount of light entering the sample.The advantage of using Absorbance is that when graphing the concentration versus Absorbance is that the relationship is linear, i.e. the graph is that of a straight line.


Absorbance is related to concentration by the Beer-Lambert law:


A = abc

isabsorbance, b is the solution path length, c is the concentration in moles per liter, and a is molar absorptivity, a constant related to the compound being tested.


Preparation of Standard Solutions:


When working with solutions, the most common method of expressing concentrations is molarity, M.Molarity is defined as moles of solute per one liter of solution, or moles per liter.Molar solutions are prepared by measuring the molar mass of the solute in grams times the desired molarity to find the number of grams of compound needed; e.g. a two molar solution of sodium chloride would require: 2 moles x 58.5 g/mol= 117 g NaCl.If volumes other than one liter are being prepared, appropriate adjustments must be made; e.g. if one is diluting the NaCl solution to 250 mL instead of one liter, only 117 g x 250/1000 = 29.25 g are needed.


Preparation of Standard Nickel Solutions:
1.
Equipment and Materials:
1.Electronic Balance
2.100 mL Volumetric Flasks
3.Nickel (II) Nitrate – Ni(NO3) 2
4.Distilled or Deionized Water

2.Procedure: 1.Calculate and measure the appropriate amount of solute needed for each standard solution.
2.Place the solute in the volumetric flask.
3.Add water to the volumetric flask until it is one-half to two-thirds full.
4.Swirl the mixture until the solute is completely dissolved.
5.Add water to the mark on the volumetric flask indicating the correct volume.
6.Cap the volumetric flask, invert and swirl the mixture to ensure that it is a completely homogeneous solution.


Formatting Requirements
1.Put your name, course and section number, and assignment title at the top of the document.
2.Use one-inch margins.
3.Use a 10-point Arial font.
4.Use double line spacing in the document.

To earn a top score, your submission should:
1.Show your calculations determining the mass of nickel (II) nitrate required to prepare 100 mL of each of the four standard solutions.
2.Include your Excel graph showing your standard calibration curve.
3.Include your final Excel graph showing the unknown in its proper place on the calibration curve.
4.Show your calculation of the mass of nickel (II) nitrate that would be found in 100 mL of the unknown solution.

[ma0641]

Now I am visually handicapped by reading your HOMEWORK! Do you really expect someone else to do all this for you?