The acid-base indicator HX undergoes the following reaction in a dilute aqueous solution: HX (color 1) H+ + X- (color 2). The following absorbance data were obtained for a 0.00035 M solution of HX in 0.1 M NaOH and 0.1 M HCI. Measurements were made at wavelengths of 450 nm and 620 nm using a 1.0 cm glass cuvette. 450 620 A(460 nm) A(630 nm) 0.1 M NaOH 0.1 M HCI 0.065 0.435 0.895 0.150 In the 0.1M NaOH solution, the indicator will be almost 100% in the X- form, while in 0.1M HCI, the indicator will be nearly 100% protonated (HX). Calculate the acid dissociation constant for the indicator if a pH=5 buffer solution containing a very small amount of indicator exhibits an absorbance of 0.567 at 450 nm and 0.395 at 620 nm (measured in a 1 cm glass cuvette).

The acid-base indicator HX undergoes the following reaction in a dilute aqueous solution: HX (color 1) H+ + X- (color 2). The following absorbance data were obtained for a 0.00035 M solution of HX in 0.1 M NaOH and 0.1 M HCI. Measurements were made at wavelengths of 450 nm and 620 nm using a 1.0 cm glass cuvette. 450 620 A(460 nm) A(630 nm) 0.1 M NaOH 0.1 M HCI 0.065 0.435 0.895 0.150 In the 0.1M NaOH solution, the indicator will be almost 100% in the X- form, while in 0.1M HCI, the indicator will be nearly 100% protonated (HX). Calculate the acid dissociation constant for the indicator if a pH=5 buffer solution containing a very small amount of indicator exhibits an absorbance of 0.567 at 450 nm and 0.395 at 620 nm (measured in a 1 cm glass cuvette).

Principles of Instrumental Analysis

7th Edition

ISBN:9781305577213

Author:Douglas A. Skoog, F. James Holler, Stanley R. Crouch

Publisher:Douglas A. Skoog, F. James Holler, Stanley R. Crouch

Chapter13: An Introduction To Ultraviolet-visible Molecular Absorption Spectrometry

Section: Chapter Questions

Problem 13.11QAP: The equilibrium constant for the conjugate acid-base pair HIn+H2OH3O++In is 8.00 10-5. From the...

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The un-ionized form of the weak acid HA does not absorb in the visible region. When it ionizes, its conjugate base (A-) absorbs light strongly at 510 nm. The following data were obtained by measuring the absorbance at 510 nm of solutions with a concen- tration of 1.0 x 104 M HA in different pH buffers. pH Absorbance 2.0 0.00 3.0 0.00 4.0 0.00 5.0 0.025 6.0 0.180 7.0 0.480 8.0 0.570 9.0 0.595 10.0 0.595 11.0 0.595 12.0 0.595 What is the approximate pK of HA?
A standard solution contains 100 ppm of Fe, which was found to have an absorbance of 0.151 au at 508 nm after the addition of 1,10-phenanthroline, pH 4 buffer, and hydroxylamine. Then, a water sample treated similarly exhibited an absorbance of 0.109 au. Which of the following statements is true? (A) The Fe concentration of the water sample is higher than 100 ppm. B) The Fe concentration of the water sample is equal to 100 ppm. It is impossible to estimate the Fe content of the water sample from information presented. D) The Fe concentration of the water sample is lower than 100 ppm.
Estimate the Ka of a weak acid from the data given below. One gram quantities of the acid were dissolved in equal quantities of the various buffer solutions and all solutions were measured under the same conditions. The anion of the acid is the only species that absorbs at the wavelength used. pH: Abs: 4 0.0 5 0.0 6 0.06 7 0.39 8 0.95 9 1.13 10 11 1.18 1.18
4. The molar absorptivities of the indicator weak acid HIn (K₁ = 1.42 x 105) and its conjugate base In at 570 were determined as ε(HIn) = 7120 and ɛ(In) = 961. The optical length b = 1.00 cm. (a) What is the absorbance of an unbuffered indicator solution having total indicator concentration 8.0 × 10%. (b) What is the absorbance of a buffered indicator solution having total indicator concentration 8.0 x 105 and pH = 6.5.
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The molar absorptivities of an indicator HIn (Ka = 4.6 × 10-5) and its conjugate base In− at 630 nm were determined as εHIn = 821 and εIn- = 375, respectively. Calculate the absorbance A (b = 1.00 cm) for the buffered indicator solution with a total HIn concentration of 6.32 × 10-4 M at pH = 4.00
Aluminium (III) reacts with the Chelating agent HL to give a complex AlL3 that is readily soluble in CHCl3. A spectrophotometric study shows that when a 1.77 x10-4 M aqueous solution of aluminium was extracted with CHCl3 containing 0.01M HL, the analytical concentration of aluminium at pH 4.5 in the two phases were 1.5 x 10-4 M [Al3+]upper) and 2.73 x10-5 M ([AlL3]lower) . What is the distribution coefficient at pH 4.8.
Treatment of ammonia with phenol in the presence of hypochlorite yields indophenol, a blue product that absrobs light at 625 nm, which çan be used for the spectrophotometric determination of ammonia. gan OCF OH + NH3 indophenol anion To determinè the ammonia concentration in a sample of lake water, three samples are prepared. In sample A, 10.0 mL of lake water is mixed with 5 mL of phenol solution and 2 mL of sodium hypochlorite solution, and diluted to 25.0 mL in a volumetric flask. In sample B, 10.0 mL of lake water is mixed with 5 mL of phenol solution, 2 mL of sodium hypochlorite solution, and 2.50 mL of a 5.50 x 10-4 M ammonia solution, and diluted to 25.0 mL. Sample C is a reagent blank. It contains 10.0 mL of distilled water, 5 mL of phenol solution, and 2 mL of sodium hypochlorite solution, diluted to 25.0 mL. The absorbance of the three samples is then measured at 625 nm in a 1.00 cm cuvette. The results are shown in the table. Sample Absorbance (625 nm) A 0.361 B 0.608 C…
If the absorbance for the solution was 0.432, calculate the equilibrium concentration of iron(III) thiocyanate in the solution. Report your answer in mM with three places after the decimal.
CHOCIS Small! X= 5.92×10-5 = [ht] plt 9) Calculate the pH of a solution that contains 1.00 MHCN (K. = 6.2 x 10-¹0) and 5.00 M HNO₂ (K₁ = 4.0 x 10). Also calculate the concentration of cyanide ion (CN) in this solution at equilibrium. ignore the HCN - the concentraction is low & Ka + is 50 Solve using a standard ICE table for tril, only
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