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 precentage of HX in sample :
A. `50%`
B. `75%`
C. `80%`
D. `90%`

Answer 1

Correct Answer - C
When half acid is neutralized pH = `pK_(a)`
At the equlivalence point
`pH = (1)/(2)[pK_(w) + pK_(a) + "log"C]`
`rArr 9 = 1/2[4+5+"log"C] = C = 0.1`
Let V ml of NaOH is used in titration milli eq. of
NaOH `= 0.25 xx V = "milli eq. of salt"`
milli equivalent or milli-moles of acid `= 10`
wt. of aicd`= 10 xx 10^(-3) xx 82 = 0.82 gm`
wt. % of acid `= (0.82)/(1.025 ) xx 100 = 80%`