Aspirin is a weak organic acid whose molecular formula is HC9H7O4. An aqueous solution of aspirin is prepared by dissolving 3.60 g/L. The pH of this solution is found to be 2.6. Calculate Ka for aspirin. (atomic mass: C

Ka = 3.50x10⁻⁴

Explanation:

First, we need to convert the unit of 3.60 g/L to mol/L:

The reaction dissociation of aspirin in water is:

C₉H₈O₄  +  H₂O  ⇄  C₉H₇O₄⁻ + H₃O⁺    

0.02 - x                        x             x

The constant of the above reaction is:

To find Ka we need to find the value of x. We know that pH = 2.6 so:

Now, the concentration of C₉H₈O₄ is:

Finally, Ka is:

Therefore, the Ka of aspirin is 3.50x10⁻⁴.

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