Video Transcript
Which of the following factors
would increase the degree of dissociation of a weak acid? (A) Increasing dilution, (B)
decreasing volume, (C) decreasing dilution, (D) increasing acid concentration.
When a weak acid is mixed with
water, only a portion of the molecules dissociate into ions. We can represent the extent to
which the molecules dissociate with the degree of dissociation. Degree of dissociation, typically
given the symbol 𝛼, is the proportion of the substance that is dissociated. The degree of dissociation is
dependent on a number of factors, including the strength of the acid, temperature,
and concentration. The strength of an acid can be
quantified using an acid dissociation constant, or 𝐾 a. The degree of dissociation, 𝐾 𝐚,
and the initial concentration of the acid, represented by 𝑐 naught, can be related
to one another using Oswald’s dilution law.
For weak acids with very low
degrees of dissociation, one minus 𝛼 can be approximated to one. And Oswald’s dilution law can be
simplified to 𝐾 𝐚 is approximately equal to 𝛼 squared times 𝑐 naught. This can be rearranged to solve for
the degree of dissociation. Here, we can see that the degree of
dissociation is inversely proportional to the square root of the acid
concentration. Thus, increasing the concentration
will decrease the degree of dissociation, and decreasing the concentration will
increase the degree of dissociation. We want to determine which factor
would cause the degree of dissociation to increase. So we need to determine which
factor would cause the concentration to decrease.
Answer choice (A), increasing
dilution means that we are adding more solvent to the solution. Increasing the amount of solvent
will cause the concentration to decrease. A decrease in concentration
corresponds with an increase in the degree of dissociation. Let’s look at answer choice
(B). Decreasing the volume of the
solvent will increase the concentration, so the degree of dissociation will
decrease. Answer choice (C) is the opposite
of answer choice (A). Decreasing dilution will cause an
increase in the concentration, so the degree of dissociation will decrease. Lastly, we know that increasing the
acid concentration will cause a decrease in the degree of dissociation. So the factor that would increase
the degree of dissociation of a weak acid is answer choice (A), increasing
dilution.
