Exp 2 new - Jonathon Nguyen 16323012 Biomolecular Dynamics...

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Jonathon Nguyen Biomolecular Dynamics 16323012 Experiment 2: Spectrophotometric Determination of Acidity Constant “A Questions: 1) What quantity is the spectrophotometric analysis measuring? A spectrophotometer measures the intensity of light as a function of the wavelength of light. The spectrophotometer measures the absorbance (A) by measuring the amount of light absorbed when the light is shined through the sample and. 2) Show how the value of the Ka for the m-nitroaniline maybe derived from this quantity A = log (I O ÷ I) NB: when Io = 0 it’s when the acid concentration at its highest concentration. This formula is derived from the pKa formula: pka = -log [H + ] [B] [BH + ] 3) Define the quantities and units of “absorbance” and “molar absorption coefficient”. “Absorbance” (A) is described as: A λ = -log 10 (I ÷I O ) I: intensity of light at a particular wavelength ( λ ) that has transmitted light intensity. IO: the initial intensity of light before it enters the sample also known as incident light 1 | P a g e
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J onathon Nguyen Biomolecular Dynamics 16323012 However, the molar absorption coefficient ( ε ) measures the strength of species to determine its absorption of light at a given wavelength. ε is mainly measured in M -1 cm -1 or LMol -1 cm -1 . 4) A solution containing 4.48ppm KMnO 4 has a transmittance of 0.309 in 1.00cm cell at 520nm. Calculate the molar absorption coefficient of KMnO 4 . At wt of KMnO4 = 158.036 158.036 × 100 = 158036ppm Therefore 1M of KMnO 4 is 158036ppm If 1M 158036ppm Then 4M xppm 4.48ppm = 2.834797 × 10 -5 M Now A = -log 10 T = ε Cl = -log 10 (0.309) = 0.51004152 Therefore ε Cl = 0.51004152 ε = 0.51004152 ....... 2.834797 × 10 -5 M Therefore ε = 1.80 × 10 -4 is the molar coefficient of KMnO 4 . Aim: 2 | P a g e
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Jonathon Nguyen Biomolecular Dynamics 16323012 1. To master the use of a spectrophotometric technique to measure the pKa of the weak acid m-nitroaniline(MNA). 2. To demonstrate the effects of Sodium chloride (inert electrolyte) on the degree of
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Exp 2 new - Jonathon Nguyen 16323012 Biomolecular Dynamics...

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