72) Amino acids are building blocks of proteins. These compounds contain at leas

Question

72) Amino acids are building blocks of proteins. These compounds contain at least one amino group N and one carboxyl group (-COOH). Consider glycine (NH CH COOH). Depending on the pH of the solution, glycine can exist in one of three possible forms: Fully protonated NH,-CH-COOH Dipolar ion: NH, CH COO Fully ionized: NH CH-Coo Predict the predominant form of elycine at p 1.0, 7.0, and 12.0. The phi of the carboxyl group ht of the ammonium group NH5) is 9.6 73) A 25.0-ml of 0.20 M HF solution is titrated with a 0.20 MNaOH solution. Calculate the volume of NaOH solution added when the pH of the solution is (a) 2.85, (b) 3.15, (c) 11.89. Ignore salt hydrolysis. (Hint: The K, value for HF is 7.1 10; hence, pka 3.15) 74) One way to distinguish a buffer solution with an acid solution is by dilution. (a) Consider a buffer solution made of 0.500 M CH COOH and 0.500 (b) Compare the result in (a) with the pHs of a 0.500 MCH,COOH solution before and after it has been diluted 10-fold. the pH after the addition of each of the following volumes of NaOH: (a) 0.0 mL (b) 10.0 mL., (c) 20.0 ml, (d) 40.0 mL, (e) 60.0 mL. A Using the available data points, plot the pH titration curve for the above titration. 76) A 40.0 ml. sample of 0.100 M acetic acid (CH CoofD mL, (e) 60.0 ml. A plot of the pli of a solution as a function of the volume of added titrant is known as a ph added titrant is known as a pi titration curve. of 0.100 M acetic acid (CH,COOH) is titrated with 0.100 M NaoH. Calculate the plH addition of each of the following volumes of NaOH: (a) 0.0 mL, (b)20.0mL, ()30.0 mL, (d) 40.0 titration curve. Using the available data points, plot the ph titration curve for the above titration. (Ka ICH,COOH 77) Calculate the pH of a buffer solution that is 0.20 M in HCN and 0.12 M in NaCN. Will the pH change if the solution is diluted by a factor of 2? Explain. K, for HCN is 4.9 x 10- tration of a strong acid is not substantially higher than 1.0x 107 M, the ionization of water must be taken into account in the calculation of the solution's pH. (a) Derive an expression for the pH of a strong acid solution, including the contribution to [H'] from H:0. (b) Calculate the pH of a 1.0x 10 MHCI solution. 79) When a typical diprotic acid H2A (K-1.2 x10+ K-21x 10-10) is titrated with NaOH, the principal A-containing species at the first equivalence point is HA". By considering all four proton-transfer reactions that can occur in an aqueous solution of HA", show that the principal reaction is 2 HA-hA+ A. (Hint: calculate the equilibrium constants for all the four proton-transfer reactions and show the principal reaction having the largest equilibrium constant among the four possible reactions.) Calculate the pli at the first equivalence point. 80) Problems in the Homework Assignments for Chapters 14, 15 and 16.

Explanation / Answer

72) Predominant form of glycine at

pH 1.0 is Fully protonated

pH 7.0 is dipolar ion

pH 12.0 is fully ionized

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73) HF/F- (acid/base)

using hendersen-Hasselbalck equation,

pH = pKa + log(F-/HF)

initial moles HF = 0.20 M x 25 ml = 5 mmol

let x amount of NaOH added

(a) pH = 2.85

2.85 = 3.15 + log(x/(5-x))

2.5 - 0.5x = x

x = 2.5/1.5 = 1.67 mmol

volume NaOH added = 1.67 mmol/0.20 M = 8.35 ml

(b) pH = 3.15

HF remains = F- formed

volume NaOH added = 5 mmol/2 x 0.20 M = 12.5 ml

(c) pH = 11.89

11.89 = 3.15 + log(x/(5-x))

43.7 - 8.74x = x

x = 43.7/8.74 = 5 mmol

volume NaOH added = 5 mmol/0.20 M = 25 ml

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