Formic Acid Chemistry

Question

Formic acid, whose formula is HCOOH, is the active ingredient in ant bite stings. A 0.10 M aqueous solution of formic acid has a pH of 2.38. What is K_a for formic acid?

Answer 1

Hi Elijah J.,

You need to put some ICE on that ant sting! (Fire ants, in particlar, bite to latch into you, then sting repeatedly as they swivel their abdomens in a circle). But specific ant species may bite and/or sting, or even spray venom. The formic acid gives the immediate pain, and venom proteins (if any) confer possible allergic reactions. To quell the pain (works for bee stings, too) topical household ammonia, rubbed in for a moment, works a treat -- better than benzocaine or hydrocortisone creams -- it not only neutralizes the acid, but anesthetizes the puncture area.

Anyway, on to the ICE (initial - change - equilibrium):

Species Initial Change Equilibrium

H+ 10^-7 +x 10^-7 + x

HCOOH 0.1 -x 0.1 - x

Now, you know that H+ at equilibrium = 10^-2.38 from the definition of pH. That means that the 10^-7 may be neglected in H+ equilibrium condition calculations, and also that +x ~ 10^-2.38. That's also the concentration of HCOO- , isn't it (since you make one ion of each on the acid dissociation)?

So let's put it together:

Ka = [HCOO-][H+]/[HCOOH]

Ka = (10^-2.38)(10^-2.38)/(0.1-10^-2.38)

So solve that -- it isn't even a quadratic, with the approximations made -- then take - log(Ka) .

-- Cheers, -- Mr. d.