A student titrates an unknown weak acid, HA, to a pale pink phenolphthalein endpoint with 25.0 mL of 0.100 M NaOH. The student then titrates in 11.0 mL of 0.100 M HCl to the solution containing the neutralized unknown acid (HCl was delivered from a second buret). The pH of the resulting solution is pH

Hello your question is incomplete the pH of the solution is unknown hence I will take the pH of the solution be = 4.7

answer : Pka = 5   i.e. > 4.7

Explanation:

Student titrates :

Unknown weak acid ( HA ) with 25 mL of 0.100 M of NaOH ( to a pale pink phenolphthalein endpoint )  then titrates with  11.0 mL of 0.100 M HCL to the solution that contains neutralized unknown acid

Assuming pH of solution = 4.7

HA + NaOH --------> H2O + NaA

at equilibrium  point

0.1 * 25 * 1 = n * 1  ∴   = 2.5 mmoles ------ 1

also

 + HCL ---------> HA + C

at equilibrium point

= 1.1 mmoles --------- 2

The ratio of reaction 2 to 1

= 1.1 / 2.5 ≈ 1/2   ( this show that when the pH = 4.7 approximately half of the A^-  have been converted to HA )

Determine the pKa value of the solution

Given that

pH = Pka + log ( A^- / hA )

4.7 = Pka + log ( 1/2 )

therefore Pka = 4.7 - ( -0.3 ) = 5

explanation:

two years ago, when picking flowers in the mountains back of yosemite valley, i found a book. it was blotted and storm-beaten; all of its outer pages were mealy and crumbly. the paper seemed to dissolve like the snow beneath which it had been buried, however; many of the inner pages were well preserved. and although all were more or less stained and torn, whole chapters were easily readable. this is the condition of the great open book of yosemite glaciers today; its granite pages have been torn and blurred by the same storms that wasted the castaway book.

two years ago, when picking flowers in the mountains back of yosemite valley; i found a book. it was blotted and storm-beaten, all of its outer pages were mealy and crumbly. the paper seemed to dissolve like the snow beneath which it had been buried, however, many of the inner pages were well preserved. and although all were more or less stained and torn; whole chapters were easily readable. this is the condition of the great open book of yosemite glaciers today, its granite pages have been torn and blurred by the same storms that wasted the castaway book.