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An acid that contains more than one ionizable proton is a polyprotic acid. The protons of these acids ionize in steps. The differences in the acid ionization constants for the successive ionizations of the protons in a polyprotic acid usually vary by roughly five orders of magnitude. As long as the difference between the successive values of K a of the acid is greater than about a factor of 20, it is appropriate to break down the calculations of the concentrations of the ions in solution into a series of steps.
Which of the following concentrations would be practically equal in a calculation of the equilibrium concentrations in a 0.134- M solution of H 2 CO 3 , a diprotic acid: [OH − ], [H 2 CO 3 ], No calculations are needed to answer this question.
[H 3 O + ] and are practically equal
Calculate the concentration of each species present in a 0.050- M solution of H 2 S.
Calculate the concentration of each species present in a 0.010-
M solution of phthalic acid, C
6 H
4 (CO
2 H)
2 .
[C 6 H 4 (CO 2 H) 2 ] 7.2 10 −3 M , [C 6 H 4 (CO 2 H)(CO 2 ) − ] = [H 3 O + ] 2.8 10 −3 M , 3.9 10 −6 M , [OH − ] 3.6 10 −12 M
Salicylic acid, HOC 6 H 4 CO 2 H, and its derivatives have been used as pain relievers for a long time. Salicylic acid occurs in small amounts in the leaves, bark, and roots of some vegetation (most notably historically in the bark of the willow tree). Extracts of these plants have been used as medications for centuries. The acid was first isolated in the laboratory in 1838.
(a) Both functional groups of salicylic acid ionize in water, with K a = 1.0 10 −3 for the—CO 2 H group and 4.2 10 −13 for the −OH group. What is the pH of a saturated solution of the acid (solubility = 1.8 g/L).
(b) Aspirin was discovered as a result of efforts to produce a derivative of salicylic acid that would not be irritating to the stomach lining. Aspirin is acetylsalicylic acid, CH 3 CO 2 C 6 H 4 CO 2 H. The −CO 2 H functional group is still present, but its acidity is reduced, K a = 3.0 10 −4 . What is the pH of a solution of aspirin with the same concentration as a saturated solution of salicylic acid (See Part a).
(c) Under some conditions, aspirin reacts with water and forms a solution of salicylic acid and acetic acid:
i. Which of the acids, salicylic acid or acetic acid, produces more hydronium ions in such a solution?
ii. What are the concentrations of molecules and ions in a solution produced by the hydrolysis of 0.50 g of aspirin dissolved in enough water to give 75 mL of solution?
The ion HTe − is an amphiprotic species; it can act as either an acid or a base.
(a) What is K a for the acid reaction of HTe − with H 2 O?
(b) What is K b for the reaction in which HTe − functions as a base in water?
(c) Demonstrate whether or not the second ionization of H 2 Te can be neglected in the calculation of [HTe − ] in a 0.10 M solution of H 2 Te.
(a)
(b)
(c)
Solving for
x gives 1.5
10
−11
M . Therefore, compared with 0.014
M , this value is negligible (1.1
10
−7 %).
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