1. The K a of proline is 2.5 × 10 – 11 in water, 2.8 × 10 – 11 in an aqueous sol

Question

1.

The Ka of proline is 2.5 × 10–11 in water, 2.8 × 10–11 in an aqueous solution that is 28% ethanol,

and 1.66 × 10–8 in 37% aqueous formaldehyde at 25°C.

a. In which solvent is proline the strongest acid? b.Rank these compounds on the basis of their

strengths as Brønsted–Lowry bases: water, ethanol, and formaldehyde.

2.

Aspirin ?Aspirin (Figure P15.63) is an acid substance whose pKa = 3.50 at body temperature

(37°C). One standard adult aspirin contains 325 mg of the drug. If you take 1 aspirin on a full

stomach (assume a volume of 1 L and a pH of 2.0), what percent of the aspirin in your stomach

is present as the anion?

Explanation / Answer

1. a. Ka is acid dissociation constant. Greater is the value of Ka, stronger is the acid in solution. Thus among the given solvents, proline is strongest acid in, 37% aqueous formaldehyde at 25 oC.

b. strengths of Bronsted-Lowry bases (increasing order) : Ethanol < water < formaldehyde

2. concentration of aspirin = 0.325 g/180.157 g/mol x 1 L = 0.0018 M

pH = -log[H+] = 2.0

[H+] = 1 x 10^-2 M

Ka = [aspirin anion][H+]/[aspirin]

3.16 x 10^-4 = [aspirin anion](1 x 10^-2)/(0.0018)

[aspirin anion] = 5.70 x 10^-5 M

percent of anion of aspirin = (5.70 x 10^-5 x 100/0.0018) = 3.16%

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