I added 25 ml of hydrocloric acid of unknown concentration. tehn added 2 drops o

Question

I added 25 ml of hydrocloric acid of unknown concentration. tehn added 2 drops of phenolphtalein.

I added 50 ml of 1M sodium hydroxide into a burette.

The inital ph was 0.95 of the HCL.

after adding 1ml each time and recording,

1.17 ph after 1ml,

1.54 after 2ml,

12.27 after 3ml,

After doing a fine titration,

The vol of the concentration is 27.90 and ph is 11.03 when it changed (equivalance point)

A. The pH equals the pKa when half of the concentration of NaOH necessary to reach the equivalence point has been added. What is the pH when ½ of the amount of NaOH necessary to reach the equivalence point has been added (i.e. what is the pKa)? Choose the closest answer.

A.2.5

B.6.3

C.4.7

D.7.2

B. Based on the pKa that you measured, what is the Ka for the dissociation of the acid? (Ka = 10-pKa) Choose the closest answer.

A.8 * 10^-8 M

B. 1 * 10^-5 M

C.7 * 10^-3 M

D.1 * 10^-1 M

Explanation / Answer

For the given titration of HCl with NaOH

A. pH at half equivalence point would be approximately,

D. 7.2

B. Ka for the acid would be thus,

A. 8 x 10^-8

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