A solution of weak acid HA is being titrated with 0.1 M NaOH solution. End point is reached on addition of x mL of NaOH. To the above solution, (x/2) mL of 0.10 M HCl is added further. pH of the resulting solution was found to be 5.3. Determine pH at the end point if initial concentration of weak acid was 0.10 M.

Question

A solution of weak acid HA is being titrated with 0.1 M NaOH solution. End point is reached on addition of x mL of NaOH. To the above solution, (x/2) mL of 0.10 M HCl is added further. pH of the resulting solution was found to be 5.3. Determine pH at the end point if initial concentration of weak acid was 0.10 M. (A)  (B)  (C)  (D)

Tardigrade Admin · Accepted Answer

<img class="img-fluid question-image" alt="image" src="https://cdn.tardigrade.in/img/question/chemistry/43c7d45fc696007c0e188ce054e30123-.png" /> pH = pK a + log ([ A -]/[ HA ])= pK a =5.3 ⇒ K a =5 × 10-6 A -+ H 2 O leftharpoons HA + OH - K a = K a =( k w / k a )=2 × 10-9 ⇒[ OH -]=√ K h C =√2 × 10-9 × 0.05=10-8 ⇒ pOH =5 and pH =9

pH=pKa​+log[HA][A−]​=pKa​=5.3 ⇒Ka​=5×10−6 A−+H2​O⇌HA+OH− Ka​=Ka​=ka​kw​​=2×10−9 ⇒[OH−]=Kh​C​ =2×10−9×0.05​=10−8 ⇒pOH=5 and pH=9