Question

g A solid weak acid is weighed, dissolved in water and diluted to exactly 50.00 ml. 25.00 ml of the solution is taken out and is titrated to a neutral endpoint with 0.10 M NaOH. The titrated portion is then mixed with the remaining untitrated portion and the pH of the mixture is measured. Mass of acid weighed out (grams) 0.771 Volume of NaOH required to reach endpoint: (ml) 18.17 pH of the mixture (half neutralized solution) 3.6 Calculate the following 1) What is the pKa of the acid

Answer 1

pKa of the acid is 3.6

Step-by-step explanation:

When a weak acid, HX, reacts with NaOH, the conjugate base, X⁻, is produced:

HX + NaOH → X⁻ + Na⁺ + H₂O

At the half neutralized solution, [HX] = [X-]

Based on Henderson-Hasselbalch equation:

pH = pKa + log [ X⁻] / [HX]

Where pH is the pH of the buffer = 3.6

pKa is the pka of the solution

And as [ X⁻] = [HX], [ X⁻] / [HX] = 1

Replacing:

3.6 = pKa + log 1

3.6 = pKa + 0

pKa of the acid is 3.6