CHM110 Pre-Lab 4

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University of Toronto, Mississauga *

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110

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Chemistry

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Feb 20, 2024

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docx

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Table Of Reagents Reagent Physical Appearance MW (g/mol) Density (g/mL) MP/BP (°C) Hazards 0. 05 M KHP Clear, colorless liquid 204.22 1.64 295-300 May cause eye irritation, harmful if absorbed through the skin, harmful if swallowed, may cause respiratory tract irritation 0. 20 M HCl Clear, colorless liquid 36.46 1.2 110 Causes severe skin burns and eye damage, toxic if inhaled, may explode if heated, may cause respiratory irritation 0. 20 M NaOH Clear, colorless liquid 39.997 2.13 323 Causes severe skin burns and eye damage, can potentially cause death if ingested ~0.02% bromophenol blue stock solution Yellow liquid at pH values below 3.0 Blue at pH values above 4.6 670 2.2 279 Harmful, if swallowed, causes serious eye irritation with redness and itchiness, may cause skin irritation, may cause respiratory tract irritation if inhaled Procedure 1. Using a 100mL volumetric flask, transfer 5.00mL of the ~0.02 bromophenol blue stock solution into it. With the same flask, add 0.05 M potassium hydrogen phthalate which is the pH 4.0 buffer solution, and fill it until the 100.0mL mark is reached. Add the stopper to the flask and mix the solution well. 2. Acquire two clean and dry 100mL beakers and proceed to label them with the letters A and B. With a 50mL graduated cylinder, fill 50 mL of the solution prepared above into each beaker. 3. Measure the pH of solution A which will also be the same as B. Be sure to rinse the pH electrode with distilled water and dry it gently using paper tissue. 4. Obtain a cuvette to measure the absorption spectrum of solution A from 380 nm to 650 nm, in 10 nm intervals with the spectrophotometers provided at the bench. ● Note that the reference solution is the KHP solution

5. Using the data that has been collected in step 4, identify the wavelengths of the two absorption peaks, λ1 and λ2. For the following steps, one student works on solution A and the other on solution B Solution A Student: 1. Acquire three sample vials and label them A1, A2 and A3. Use the following table below to add in the reagent volumes required to prepare solutions A1, A2, and A3. A1 A2 A3 Solution A (mL) 8 8 16 0. 05 M KHP (mL) 2 1.6 0 0. 20 M HCl (mL) 0 0.4 4 Total Volume (mL) 10 10 20 2. The solution in A3 is the bromophenol blue in its acid form (Hln). Acquire three large test tubes and label them A4, A5 and A6 and make dilutions of the A3 solution. 3. Add solution A3, and 0.20 M HCl to each test tube. Use the table below to follow the dilutions required to prepare. A4 A5 A6 % A3 in Mixture 75 50 25 A3 (mL) 7.5 5 2.5 0. 20 M HCl (mL) 2.5 5 2.5 Total Volume (mL) 10 10 10 4. Measure the: ● pH of A1, A2 and A3 ● absorbance of A1 and A2 at λ1 and λ2 ● Spectrum of A3 from 380 to 650 nm ● absorbance of A4, A5, and A6 at λ1 and λ2 Solution B Student: 1. Get three sample vials and label them B1, B2 and B3. Use the following table below to add in the reagent volumes required to prepare solutions B1, B2, and B3.

B1 B2 B3 Solution A (mL) 8 8 16 0.05 M KHP (mL) 2 1.8 0 0.20 M HCl (mL) 0 0.2 4 Total Volume (mL) 10 10 20 2. Acquire three sample vials and label them B1, B2, and B3. Add solution B3, 0.05 M KHP, and 0.20 M NaOH to each vial as followed by the table below. B4 B5 B6 % A3 in Mixture 75 50 25 B3 (mL) 7.5 5 2.5 0. 20 M HCl (mL) 2.5 5 7.5 Total Volume (mL) 10 10 10 3. Measure the following: ● pH of solutions B1, B2, and B3 ● the absorbance of solutions B1 and B2 at λ1 and λ2 ● the absorbance of solutions B4, B5, and B6 at λ1 and λ2 4. After the experiment, clean up by: ● removing the cuvette from the spectrophotometer empty and rinse it twice with distilled water. ● turning off the spectrophotometer ● dispose all solutions that was prepared and used during the experiment into the liquid waste pails in the common fume hoods ● wash the volumetric flask and leave it on the bench ● wash the sample vials and the large test tubes and return them to the station ● return all glassware back into the locker Pre-lab Questions 1. The acid form of bromophenol blue (HIn) is yellow; the base form (In−) is blue. Approximately what wavelength of visible light do you think each of these forms will absorb? Briefly explain.

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For bromophenol blue, the yellow color of its acid form means that it absorbs light in the indigo color part of the visible spectrum which is around around 425–450 nm absorbance as indigo is its complementary colour. As for its base form which is blue, the base form should absorb the complementary color which is orange. Thus, the base form will absorb the orange light which has a wavelength of 590-620 nm. 2. Solution A3 is considered to be the acid form of the indicator. Solution B3 is considered to be the basic form of the indicator. Consider Le Chatelier’s Principle and explain why this is so. According to the Le Chatelier’s principle, it states that if the system at equilibrium is disturbed whether it be the concentration, temperature or pressure, then the system will shift to establish a new equilibrium. Since a strong base like NaOH was added in B3, the OH concentration will increase and it will react with the H3O to produce water. This reduces the H3O concentration and this will shift the equilibrium to the right to produce more of the base form In. As In reacts with water, the OH ions concentration will increase to the right, which will shift the equilibrium to the left to produce more In. This makes B3 the base form of the indicator. Whereas for the solution A3, the strong acid which is HCl had been added which will produce H3O ions which in this case would shift the equilibrium the left to produce more of the acid form of the indicator. The H3O reacts with the OH ions to produce water and reduces the OH concentration. This means that the equilibrium is shifted to the right producing more HIn. Hence A3 is considered the acid form of the indicator.