Back
- Many moderately large organic molecules containing basic nitrogen atoms are not very soluble in water as neutral molecules, but they are frequently much more soluble as their acid salts. Assuming that the pH in the stomach is 2.5, indicate whether each of the following compounds would be present in the stomach as the neutral base or in the protonated form: nicotine, Kb = 7 * 10-7; caffeine, Kb = 4 * 10-14; strychnine, Kb = 1 * 10-6; quinine, Kb = 1.1 * 10-6.
Problem 113
- The amino acid glycine H₂N¬CH₂¬COOH can participate in the following equilibria in water: H₂N¬CH₂¬COOH + H₂O ⇌ H₂N¬CH₂¬COO⁻ + H₃O⁺ with Ka = 4.3 × 10⁻³, and H₂N¬CH₂¬COOH + H₂O ⇌ +H₃N¬CH₂¬COOH + OH⁻ with Kb = 6.0 × 10⁻⁵. (c) What would be the predominant form of glycine in a solution with pH 13? With pH 1?
Problem 114
Problem 114a
The amino acid glycine (H2N–CH2–COOH) can participate in the following equilibria in water:
H2N–CH2–COOH + H2O ⇌ H2N–CH2–COO– + H3O+ Ka = 4.3 × 10-3
H2N–CH2–COOH + H2O⇌ +H3N–CH2–COOH + OH- Kb = 6.0 × 10-5
(a) Use the values of Ka and Kb to estimate the equilibrium constant for the intramolecular proton transfer to form a zwitterion: H2N–CH2–COOH ⇌ +H3N–CH2–COO–
Problem 114b
The amino acid glycine (H2N–CH2–COOH) can participate in the following equilibria in water:
H2N–CH2–COOH + H2O ⇌ H2N–CH2–COO– + H3O+ Ka = 4.3 × 10-3
H2N–CH2–COOH + H2O⇌ +H3N–CH2–COOH + OH- Kb = 6.0 × 10-5
(b) What is the pH of a 0.050 M aqueous solution of glycine?
- The pKb of water is _______. (a) 1 (b) 7 (c) 14 (d) not defined (e) none of the above
Problem 115
- Calculate the number of H3O+ ions in 1.0 mL of pure water at 25 °C.
Problem 116
- How many milliliters of concentrated hydrochloric acid solution (36.0% HCl by mass, density = 1.18 g/mL) are required to produce 10.0 L of a solution that has a pH of 2.05?
Problem 117
- The volume of an adult's stomach ranges from about 50 mL when empty to 1 L when full. If the stomach volume is 400 mL and its contents have a pH of 2, how many moles of H+ does the stomach contain? Assuming that all the H+ comes from HCl, how many grams of sodium hydrogen carbonate will totally neutralize the stomach acid?
Problem 118
Problem 119a
Atmospheric CO2 levels have risen by nearly 20% over the past 40 years from 320 ppm to 400 ppm. (a) Given that the average pH of clean, unpolluted rain today is 5.4, determine the pH of unpolluted rain 40 years ago. Assume that carbonic acid 1H2CO32 formed by the reaction of CO2 and water is the only factor influencing pH. CO21g2 + H2O1l2 Δ H2CO31aq2
Problem 120
At 50 °C, the ion-product constant for H2O has the value Kw = 5.48 * 10-14. (a) What is the pH of pure water at 50 °C? (b) Based on the change in Kw with temperature, predict whether ΔH is positive, negative, or zero for the autoionization reaction of water: 2 H2O1l2 Δ H3O+1aq2 + OH-1aq2
- In many reactions, the addition of AlCl3 produces the same effect as the addition of H+. (a) Draw a Lewis structure for AlCl3 in which no atoms carry formal charges, and determine its structure using the VSEPR method. (b) What characteristic is notable about the structure in part (a) that helps us understand the acidic character of AlCl3?
Problem 121
- Cocaine is a weak organic base whose molecular formula is C17H21NO4. An aqueous solution of cocaine was found to have a pH of 8.53 and an osmotic pressure of 52.7 torr at 15 °C. Calculate the Kb for cocaine.
Problem 124
- The iodate ion is reduced by sulfite according to the following reaction: IO3^- (aq) + 3 SO3^2- (aq) -> I^- (aq) + 3 SO4^2- (aq). The rate of this reaction is found to be first order in IO3^-, first order in SO3^2-, and first order in H+. (b) By what factor will the rate of the reaction change if the pH is lowered from 5.00 to 3.50? Does the reaction proceed more quickly or more slowly at the lower pH? (c) By using the concepts discussed in Section 14.6, explain how the reaction can be pH-dependent even though H+ does not appear in the overall reaction.
Problem 125
- Using dissociation constants from Appendix D, determine the value for the equilibrium constant for each of the following reactions. (i) HCO3-1aq2 + OH-1aq2 ΔCO32-1aq2 + H2O1l2 (ii) NH4+1aq2 + CO32-1aq2 ΔNH31aq2 + HCO3-1aq2
Problem 126