Question

The active ingredient formed by aspirin in the body is salicylic acid, C₆H₄OH(CO₂H). The carboxyl group (−CO₂H) acts as a weak acid. The phenol group (an OH group bonded to an aromatic ring) also acts as an acid but a much weaker acid. List, in order of descending concentration, all of the ionic and molecular species present in a 0.001-M aqueous solution of C₆H₄OH(CO₂H).

a) C₆H₄OH(CO₂H), C₆H₄OH(CO₂⁻H), H⁺, OH⁻
b) C₆H₄OH(CO₂H), C₆H₄OH(CO₂⁻H), H⁺, H₂O
c) C₆H₄OH(CO₂⁻H), H⁺, OH⁻, H₂O
d) C₆H₄OH(CO₂⁻H), H⁺, H₂O, OH⁻

Answer 1

In a 0.001-M aqueous solution of salicylic acid, the species present in descending order of concentration are the undissociated acid (C6H4OH(CO2H)), the ionized acid (C6H4OH(CO2−)), the hydronium ion (H+), and a very small concentration of hydroxide ions (OH−).

Step-by-step explanation:

The active ingredient formed by aspirin in the body is salicylic acid, C6H4OH(CO2H), which can dissociate into various ionic and molecular species in aqueous solution.

Given the compound's structure, the carboxyl group (–CO2H) acts as a weak acid, and the phenol group (–OH attached to an aromatic ring), acts as an even weaker acid.

In a 0.001-M aqueous solution of C6H4OH(CO2H), the species present, starting from the highest concentration, would be the undissociated salicylic acid (C6H4OH(CO2H)), followed by the ion formed when the carboxyl group loses a proton (C6H4OH(CO2−)), then the hydronium ion (H+), and lastly, due to the water's autodissociation, a very small amount of hydroxide ions (OH−).

Since salicylic acid is a weak acid, most will remain undissociated, with a smaller portion present in its ionized form, and very small amounts of H+ and OH− ions.