In an ideal case, the acid would completely protonate. such an acid would be a strong acid. Dissociation of Formic acid looks like:
HCOOH + H₂O → COOH⁻ + H₃O⁺
one mole of formic acid should give one mole of Hydronium(H₃O⁺) ions
similarly, 0.1 moles of formic acid should produce 0.1 moles of hydronium ions. but we know that it is not the case, a much lesser amount is actually formed.
Which means that Formic acid did not completely dissociate into COOH⁻ and H⁺ ions.
Hence, Formic Acid is a weak acid
