Which of the following acid is stronger than benzoic acid Ka=6.3×10−5?

∵pKa=−log⁡Ka ∴pKa for benzoic acid =−log⁡6.3×10−5 =−log⁡6.3−log⁡10−5 =−0.799+5=4.201  (a) pKa for A=−log⁡1.67×10−8 =−log⁡1.67−log⁡10−8 =−0.222+8=7.778  (b) pKa for B=6,   (c) pKa for C=4.0  (d) pKa for D=−log⁡1.0×10−5 =−log⁡1.0−log⁡10−5=0+5=5For a stronger acid, pKa must be less. Therefore, acid C will be stronger than benzoic acid.