Answer: Consider the equilibrium of the generalized acid, HX , in aqueous solution….
HX(aq)+H2O(l)⇌H3O++X−
As we FACE the page, the given reaction, with FORMIC acid, HC(=O)OH , produces ONLY millimolar concentration with respect to hydronium ion, and acetate ion … and thus, as anticipated, formic acid is a WEAK acid…. pKa=3.79 .
i.e. Ka=(0.004∙mol∙L−1)20.10∙mol∙L−1−0.004∙mol∙L−1=1.60×10−4
pKa=−log10(1.60×10−4)=−(−3.79)=+3.79
….formic acid is nevertheless a STRONGER acid than acetic acid, for which pKa=+4.76 .
Explanation: