Video Transcript
Which of the following statements
best describes the degree of dissociation of an acid? (A) The fraction of reactant
molecules that dissociate at equilibrium. (B) The fraction of ionic species
produced at equilibrium. (C) The probability that the acid
will dissociate in an aqueous solution. (D) The concentration of the acid
at equilibrium. (E) The total number of hydrogen
atoms that can dissociate from a molecule.
Degree of dissociation, represented
by the symbol 𝛼, can be used to indicate the extent to which an acid dissociates
when dissolved in water. The degree of dissociation can be
calculated by dividing the amount of dissociated substance by the total amount of
substance both dissociated and undissociated, where the amounts are given as either
a number of molecules or a number of moles.
Let’s consider 50 molecules of
hydrochloric acid, a strong acid. When the hydrochloric acid is
dissolved in water, all 50 molecules will dissociate into hydrogen ions and chloride
ions. The total amount of hydrochloric
acid was 50 molecules and the number of molecules that dissociated was 50
molecules. So, hydrochloric acid has a degree
of dissociation of one.
Now let’s consider 50 molecules of
hydrofluoric acid, a weak acid. When the hydrofluoric acid is
dissolved in water, on average, only about one molecule will dissociate into
hydrogen ions and fluoride ions. The total amount of hydrofluoric
acid was 50 molecules. And the number of molecules that
dissociated was one molecule. So, hydrofluoric acid has a degree
of dissociation of 0.02.
From our examples, we can see that
the degree of dissociation can be represented as a fraction that represents how many
of the reactant molecules have dissociated at equilibrium.
Looking at the answer choices, this
is best represented by answer choice (A). So, the statement that best
describes the degree of dissociation of an acid is answer choice (A), the fraction
of reactant molecules that dissociate at equilibrium.
