Lab 4- Molarity, Dilution, and Preparing Solution-1

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CHEM 1412 Lab – 4 Dr. Pahlavan - 1 - Molarity, Dilutions, and Preparing Solutions (Measuring Absorbance using Spectrophotometer) Please Watch This Video: https://www.youtube.com/watch?v=J3cjoTMxBkk Purpose: a) To prepare solution and determine the amount of copper using a spectrophotometric method. b) Determine the relationship between the concentration and the absorption of light by the solution using the spectrophotometer. c) To demonstrate the operation of a spectrophotometer and to determine whether Beer's Law applies to a particular solution. Equipment and Chemicals Balance Plastic pipettes Spectrophotometer - 20 Cuvettes Graduated cylinder, (10-mL) Spatula Test tube rack Test tubes Volumetric flask (100-mL) Weighing paper Copper (II) sulfate pentahydrate, CuSO 4 • 5H 2 O Introduction Solutions are in an important part of chemistry and must be prepared with accuracy. In this laboratory a copper (II) sulfate solution prepared, and then will be diluted to other solutions with different concentrations. A a concentrated solution contains a large amount of solute in each amount of solution and dilute solution contains only a small amount of solute in a given amount of solution. To describe the concentration of a solution expressed in molarity, M, which is defined as the number of moles of solute in one liter of solution ( Eq. 1 ). Example 1 - Consider the preparation of 500.0mL of a 0.50M solution of sodium chloride: g (NaCl)= (100.0ml x1.0 L/1000ml) x ( 0.50moles NaCl) ) ( 58.5 g NaCl ) = 2.925 ~ 3.00 g NaCl 1L NaCl 1mole NaCl To prepare 100.0mL of 0.50M NaCl solution. Using analytical balance to measure 3.00 g of NaCl and add it to 100 ml volumetric flask and dilute it with deionized water (D.I.) to the mark. For a 100-mL volumetric flask, the mark on the neck indicated that when filled to the mark, the flask will contain exactly 100.0 mL at of 0.50M solution of NaCl.

CHEM 1412 Lab – 4 Dr. Pahlavan - 2 - Diluting Solutions To make a dilution, the following equation is used: M 1 V 1 = M 2 V 2 In this equation, M 1 is the concentration of the stock solution, V 1 , is the volume of the stock solution required to prepare the dilute solution, M 2 is the concentration of the desired dilute solution, and V 2 is the volume of the dilute solution needed. The dilution equation is commonly written as shown in Eq. 2 . The subscripts 1 and 2 refer to the concentrated solution and the dilute solution, respectively. M 1 V 1 = M 2 V 2 Eq. 2 Example 2 - Assume that the 0.50M sodium chloride solution prepared in the example above is in the stockroom, but for another experiment, 100mL of a 0.10M sodium chloride solution is needed. In performing a dilution calculation, M 1 , M 2 , and V 2 are generally known, and Equation 2 is rearranged to solve for the unknown V 1 . Using equation 1 allows us to solve for the volume of the concentrated solution required to prepare the dilute solution. Proper analytical technique for preparing the diluted solution requires that the initial and final volumes (V 1 and V 2 ) must be accurately measured using a graduated cylinder or, preferably, a pipette and a volumetric flask. Concentration and Absorbance , Beer’s Lambert Law An important problem chemist encounter in the lab is how to determine the concentration of an unknown solution. If the solution is colored, the concentration of an unknown solution can be determined by measuring the intensity of the color. A colorimeter (spectrophotometer) is used to measure the absorbance of visible light that gives the solution its color. Generally, the more intense the color of the solution, the greater the absorbance of light will be. In using colorimetry, it is important to remember that the color of light transmitted by the solution (the color we see) is complimentary to the color of light absorbed by the solution (the color we measure). Since the color of light depends on its wavelength, the wavelength of light absorbed by a colored substance in solution is complementary to the wavelength of light transmitted by the substance. Copper (II) sulfate solutions, for example, are blue. The absorbance of copper (II) sulfate solutions is measured at a wavelength of 635nm , corresponding to red light. A spectrophotometer essentially enables one to isolate a narrow beam of nearly monochromatic (one wavelength) light, pass it through a sample, and measure the radiant power, I, of the light transmitted through the sample relative to the incident radiant power, I o . The ratio I/I o is called the transmittance , T, and is the fraction of light of a given wavelength transmitted through the sample. The percent transmittance is given by: %T = (I/I o ) x 100 Beer's law states that, for a given solution, the absorbance is directly proportional to path length and solute concentration. In the spectrophotometric measurement of color intensity, the light transmitted through the solution is measured.

CHEM 1412 Lab – 4 Dr. Pahlavan - 3 - Another term that is more useful theoretically than transmittance is absorbance. It is a measure of the amount of light of a given wavelength absorbed by the sample itself and is thus related to the interaction of the light with the atoms, molecules, and ions in the sample. As expected from their definitions, absorbance, A, and transmittance are inversely related. The mathematical relationship is: A = log (1/T) = -log T and A = 2 – log (%T) Example 3 – Do the following conversions. a) absorbance 0.015 to transmittance ➔ T = 100 antilog (-0.0150) = 96.6% b0 25.5% transmittance to absorbance ➔ A = - Log (25.5/100) = 0.593 OR A = 2 - Log (25.5) = 0.593 S pectrophotometer shows both percent transmittance ( %T) and absorbance ( A ). The amount of light at a given wavelength that a sample absorbs is often directly proportional to the concentration of the absorbing species in the sample and to the distance (path length) the light must travel through the sample. Mathematically, this relationship is given by Beer’s Law and is expressed as: Example 4- A compound had a molar absorptivity of 2.17x10 3 L/cm.mol. What concentration of compound would be to produce in solution having transmittance of 18.24% in a 2.50 cm cell? a = 2.17x103 L/cm.mol , T% = 18.24% , b = 2.50 cm , c = ? Solution: A = -Log (T%) = -Log (18.24/100) = 0.735 A = abc ➔ c = (A /ab) = (0.735) / (2.17x103 x 2.50) ➔ c = 1.35x10 -4 mol/L or M The following is the schematic presentation of spectrophotometer ( Fig 1 ). and absorbance vs. concentration graph ( Fig. 2 ).

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CHEM 1412 Lab – 4 Dr. Pahlavan - 4 - Working Curve Analysis The absorbance of a series of three to five standard solutions are measured and plotted on graph paper against the concentrations of these standards. This is known as a Beer's Law plot. The absorbance of an unknown concentration is then measured, and its concentration is determined directly from the plot. This method is more commonly used than the absolute calculation, because experimental error will average out over the number of standards. and absorbance vs. concentration graph ( Fig. 2 ). Fig. 1 Fig. 2 S afety Precautions Copper (II) sulfate is moderately toxic by ingestion and inhalation and is a skin and respiratory irritant. Avoid contact with eyes and skin. Wear chemical splash goggles, chemical resistant gloves, and a chemical- resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory.

CHEM 1412 Lab – 4 Dr. Pahlavan - 5 - Procedure Part A. Preparing the Stock Solution 1. Calculate the number of grams of copper (II) sulfate pentahydrate, CuSO 4 • 5H 2 O, required to prepare 100.0 mL of a 0.250 M solution. Once your calculations have been approved by your instructor, weigh out the required amount of copper sulfate on a balance in a clean, dry weighing paper. 2. Transfer the solid to a 100-mL volumetric flask. Add distilled or deionized water and mix the solution to give a homogeneous solution to the mark line on flask. Label that “S” for stock solu tion. Part B. Preparing Diluted Solutions 1. Select five clean and dry test tubes in a test tube rack and label them 1 through 5. Using a 10mL graduated cylinder, measure and pour 10mL of the 0.250M stock solution into test tube #1. Record the necessary data for this solution in the data table. 3. Fill the 10-mL graduated cylinder exactly to the 2.00-mL mark with the stock solution and fill the graduated cylinder to the 10.0-mL mark with distilled or deionized water. 4. Repeat same dilution with 3mL, 4 mL, and 5 mL for test tubes 3, 4, and 5. 5. Proceed your calculations for preparation of 10.0mL cupric sulfate solution 1, 2, 3, 4, and 5. Check with your instructor before proceeding. record the necessary data in the data table. 6. Compare the color of the stock solution and each of the dilutions in test tubes #1-5. Record these observations in the data table.

CHEM 1412 Lab – 4 Dr. Pahlavan - 6 - Part C. Colorimetry Measurements Using Spectrum- 20 (Spectrophotometer) 1. Turn on the spectrophotometer and allow it to warm up for 5-10 minutes. 2. Turn the wavelength knob and adjust the wavelength to 635nm. 3. Fill a cuvette about ¾ full will distilled water. Then place the cuvette in the colorimeter compartment. and adjust to read 0%T and 0 absorbance with a “blank” cuvette containing only distilled water. 4. Fill the cuvettes with each solution. Wipe the cuvettes with paper. 5. Record the absorbance data for solutions #1-5 in the Data Table. 6. Prepare a graph of absorbance on the y-axis versus the concentration of each solution on the x-axis. 7. Obtain an absorbance reading for the unknown sample. 8. Make a graph of concentration, c, (x-axis) vs. absorbance, a, (y-axis). 9. From the standard curve, determine the concentration of copper in unknown sample solution..

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CHEM 1412 Lab – 4 Dr. Pahlavan - 7 - REPORT FORM Molarity, Dilutions, and Preparing Solutions Test Tube 1 2 3 4 5 Volume of Stock Solution (V 1 ) 1.0mL 2.0mL 3.0mL 4.0 mL 5.0 mL Concentration of Stock Solution (M 1 ) 0.250M 0.250M 0.250M 0.250M 0.250M Final Volume of Diluted Solution (V 2 ) 10.0mL 10.0mL 10.0mL 10.0mL 10.0mL Concentration of Diluted Solution (M 2 ) (show Calculation) M 2 = (M 1 V 1 )/V 2 Absorbance at 635 nm (A) Percent Transmittance (%T) Unknown Identification: _______ Absorbance (A) = ____________ Concentration (c): ____________

CHEM 1412 Lab – 4 Dr. Pahlavan - 8 - Molarity, Dilutions, and Preparing Solutions Pre- laboratory Questions and Exercises Name __________________________ Due before lab begins. Answer on a separate sheet of paper. Date ___________________________ 1. Convert the accompanying transmittance data to absorbance. a) 53.86% b) 0.567% 2. Express the following absorbances in terms of percent transmittance. a) 0.918 b) 0.072 3. What is the relationship of percentage transmittance and absorbance? If the percentage transmittance for a sample is 100 at a given wavelength, what is the value of absorbance? 4. Calculate the molar concentration of a solution if the absorbance (A) is 0.244, the molar absorptivity (A) is 3.39 x 10 4 L/mole cm and the path length of the cell (b) is 2.00 cm.

CHEM 1412 Lab – 4 Dr. Pahlavan - 9 - Molarity, Dilutions, and Preparing Solutions Post- laboratory Questions and Exercises Name _____________________ Due after completing lab. Answer in the space provided. Date ______________________ 1. Calculate the number of grams of copper (II) sulfate pentahydrate, CuSO 4 • 5H 2 O, required to prepare 100.0mL of the following solution by dilution process. a) 0.200 M copper (II) sulfate solution. b) Calculate the number of milliliters of 0.250M copper (II) sulfate solution that must be diluted to prepare 10.0mL of a 0.150M copper (II) sulfate solution. 2. For a solution of 0.200 M Cu (NO 3 ) 2 in a 1-cm cell, the following data has been collected. Calculate the percentage transmittance, %T, for each absorbance. plot A vs. C and T% vs. C on the same graph. C(mol/L) A % T 0.010 0.112 0.030 0.336 0.060 0.672 0.090 1.008 0.120 1.120

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