If you dissolve 0.1 mol of formic acid in 1 L of water, ther esulting solution contains 0.004 mol of H₃O+. Based on this information, is formic acid a strong acid, or is it a weak acid?

 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:

a