In the titration of a solution of the extremely weak acid, HCN (Ka = 6.2 x 10-10
ID: 861259 • Letter: I
Question
In the titration of a solution of the extremely weak acid, HCN (Ka = 6.2 x 10-10), by a solution of sodium hydroxide of equal concentration, which of the following statements is true?
a. The equivalence point will occur at an acidic pH, and a suitable indicator would be one like alizarin that changes color between pH of 6 and about 7
b. The equivalence point will occur at a neutral pH, and a suitable indicator would be one like bromthymol blue that changes color between pH of 6 and about 7.5
c. The equivalence point will occur at a basic pH, and a suitable indicator would be one like bromthymol blue that changes color between pH of 6 and about 7.5
d. The equivalence point will occur at a basic pH, and a suitable indicator would be one like phenol red that changes color between pH of 7 and about 8
e. The equivalence point will occur at a basic pH, and a suitable indicator would be one like phenolphthalein that changes color between pH of 8 and about 11.
Please explain the answer for the points
Explanation / Answer
Ph = -logKa = 10 - log6.2 = 9.2
Phenolpthalein is another weak acid and turns the weaker acidic ion (CN-) solution into bright pink