Question

A 1.025 g sample containing a weak acid HX (mol. mass = 82) is dissolved in 60 mL water and titrated with 0.25 M NaOH. When half of the acid was neutralised the pH was found to be 5.0 and at the equivalence point the pH is 9.0. Calculate mass percentage of HX in sample :

• A. 50%
• B. 75%
• C. 80%
• D. None of these

Answer 1

Answer: (C)

80%

Answer 2

The $pK_a$ of the weak acid is determined from the $pH$ at the half-neutralization point. The $pH$ at the equivalence point is used to find the concentration of the salt formed. Using this concentration, the volume of titrant ($NaOH$) required for equivalence is calculated. From the volume of titrant and its molarity, the moles of the weak acid are found. Finally, the mass percentage of the weak acid in the sample is computed.