Values for Ka for three weak acids are given below, at T = 25 C, along with their conjugate bases
formic acid (HCOOH) Ka = 1.7 x 10-4 conjugate base = HCOO-
hypoiodic acid (HIO) Ka = 2.3 x 10-11 conjugate base = IO-
phenol (C6H5OH) Ka = 1.3 x 10-10 conjugate base = C6H5O-
Based on this information, we can say
a) HCOO- is a stronger base than IO-
b) HCOO- is a stronger base that C6H5O-
c) IO- is a stronger base than C6H5O-
d) Both a and b
e) All three conjugate bases are the same strength
According to Bronsted -Lowry concept
if the acid were stronger its conjugate base is weak. The higher the Ka value the stronger is the acid.
Thus let us arrange the acids in decreasing order of strength using their Ka values.
formic acid > phenol > HIO
Thus the conjugate bases have the strength in reverse order
HCOO- < C6H5O- < IO-
Thus only option C is correct ( IO- is stronger base than both C6H5O- and HCOO-) as it is the conjugate base of weakest acid HIO
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