You determine that it takes 26.0 mL of base to neutralize a sample of your unkno

Question

You determine that it takes 26.0 mL of base to neutralize a sample of your unknown acid solution. The pH of the solution when exactly 13 mL of base had been added was 4.18. You notice that the concentration of the unknown acid was 0.1 M.

a. What are the Ka and the pKa of your unknown acid? (Show work)

b. What it the probable identity f your unknown acid? (use chart in discussion)

c. After finishing your calculations, you discover that the actual concentration of the unknown acid was 0.27 M. How would this affect your calculation of the pKa?

Explanation / Answer

given 26 ml takes to equivalence point

at equivalence point

moles of acid = moles of base

now only 13 ml of base is added

so only half of the acid will be reacted and half remains

the remaining acid and salt forms a buffer soltuion

acid + base ---> salt + water

pH = pKa + log[ salt / acid ]

from the balanced eqaution

moles of salt formed = moles of acid remaining

so

pH = Pka + 0

pH = pKa

4.18 = pKa

so

pKa = 4.18

it is a general concept that

at half equivalence point pH = pKa

pKa = -log Ka

4.18 = -log Ka

Ka = 6.6 x 10-5

b ) it may be benzoic acid

c) 0.27 > 0.1

so the equivalence point should be greater than 26 ml .

so 13 ml is not half equivalent point. it should be more than 13 ml

so the Pka should be more than the obtained value of 4.18

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