Problem: Phenylacetic acid (HPAc) is a modified form of an amino acid phenyl alanine seen in patients with phenylketonuria, an inherited disorder. A study of HPAc shows that the pH of a 0.12 M HPAc solution is 2.62. What is the Ka of HPAc?A. 2.4 × 10–3B. 4.8 × 10–5 C. 3.6 × 10–8 D. 5.0 × 10–4E. 1.2 × 10–5

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FREE Expert Solution

We’re being asked to calculate the equilibrium constant of HPAc if a 0.12 M solution has a pH of 2.62


Remember that weak acids partially dissociate in water and that acids donate H+ to the base (water in this case). 


The dissociation of HPAc is as follows:

HPAc(aq) + H2O(l)  H3O+(aq) + PAc(aq)


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Problem Details

Phenylacetic acid (HPAc) is a modified form of an amino acid phenyl alanine seen in patients with phenylketonuria, an inherited disorder. A study of HPAc shows that the pH of a 0.12 M HPAc solution is 2.62. What is the Ka of HPAc?


A. 2.4 × 10–3

B. 4.8 × 10–5 

C. 3.6 × 10–8 

D. 5.0 × 10–4

E. 1.2 × 10–5