Video Transcript
Calculate the value of the acid
dissociation constant 𝐾 𝑎 for methanoic acid, HCOOH, using the following two
values. The concentration of HCOOH aqueous
equals 1.5 times 10 to the negative second moles per liter and the concentration of
H+ aqueous equals 1.5 times 10 to the negative third moles per liter. You can assume that the
concentration of H+ aqueous equals the concentration of HCOO minus aqueous. Give your answer to one decimal
place and in scientific notation.
An acid dissociation constant is
the equilibrium constant for the reaction of an acid with water. When a weak acid reacts with water,
we can simplify the reaction to say that the acid dissociates into ions as shown
here in this generic reaction equation.
The acid dissociation constant for
a weak acid can be calculated by multiplying the equilibrium concentrations of H+
and A− ions, then dividing by the equilibrium concentration of the acid. The weak acid in this question is
methanoic acid. It dissociates in water to produce
hydrogen ions and methanoate ions. To calculate the acid dissociation
constant for methanoic acid, we’ll need to multiply the equilibrium concentrations
of H+ and HCOO−, then divide by the equilibrium concentration of HCOOH.
We are given the concentrations of
methanoic acid and hydrogen ions. We are also told to assume that the
hydrogen ion and methanoate ion concentrations are equal. We can substitute all of these
concentrations into the equation. We can see that some of the moles
per liter units will cancel such that 𝐾 𝑎 will have the unit moles per liter. Performing the calculation gives us
a value of 0.00015 moles per liter.
In scientific notation and to one
decimal place, we have determined that the acid dissociation constant for methanoic
acid is 1.5 times 10 to the negative fourth moles per liter.
