Answer:
The unknown compound is a weak acid.
Step-by-step explanation:
Given that :
a 25 mL sample of a solution of an unknown compound is titrated with a 0.115 M NaOH solution.
A buffer region was found around a pH of 3.5. We know that a pH of 3.5 is a weak acid. So, it is likely to be an organic acid
Let assume the solution of the unknown sample to be CH₃COOH
Now :
25 mL of CH₃COOH reacted with 0.115 M of NaOH
The equation for the reaction will be :
CH₃COOH + NaOH -----> CH₃COONa + H₂O
at x mole of 0.115y M of
CH₃COOH NaOH is present
If NaOH was added in excess;
CH₃COOH + NaOH -----> CH₃COONa , NaOH will be lost then CH₃COOH and CH₃COONa will be present
Therefore;
At equilibrium : Only CH₃COONa will be present but if it is above equilibrium NaOH will be present because the pH will increase due to the presence of the strong base