A 8.42-g sample of homogentisic acid,a weak organic acid having K_a = 4.0 × 10^-5,is dissolved in 35.00 mL of water and its pH is measured to be 2.122.What is the molar mass of homogentisic acid?

Question

Quizplus · Accepted Answer

The molar mass of homogentisic acid can be calculated by first determining the concentration of H+ ions in the solution from the given pH, then using the dissociation constant (Ka) to find the concentration of the acid, and finally using the mass and concentration to find the molar mass. The pH of 2.122 corresponds to a [H+] concentration of 10^-2.122 M. Given the Ka and the initial concentration, we can set up an equilibrium expression to solve for the concentration of the acid. However, since the pH and the concentration of H+ ions directly give us the amount of acid that has dissociated, and knowing the volume of the solution allows us to calculate the moles of acid, we can directly calculate the molar mass. The molar mass is found by dividing the mass of the acid sample by the moles of acid, which gives us 168 g/mol as the correct answer.

A 842-G Sample of Homogentisic Acid,a Weak Organic Acid Having Ka

A 842-G Sample of Homogentisic Acid,a Weak Organic Acid Having K_a