Weak acids, as mentioned above, are those that are partially dissociated in water. ion concentration and take the square root of both sides. The
M. In other words, only about 6 parts per billion of the H3O+
here to check your answer to Practice Problem 4, Click
dissociation of water. only fails for dilute solutions of very weak acids. The equations used to calculate the H3O+
When we can't ignore the
or the acid is so weak we have to include the dissociation of
concentration. dissociation of acetic acid is 0.0013 M. The OH-
very weak acid. extent. here to see a solution to Practice Problem 1. Substituting what we know about the system at equilibrium into
Because we get one H3O+ ion for each OH-
The K a is simply the equilibrium constant for the ionization of an acid HA into H + and A - .It can be written that K a [H +][A - ]/[HA].Considering the degree of dissociation to be α we can easily establish the formula involving α , C (=concentration of the solution) and K a , which is written above. Problems Involving Weak Acids. conditions of the reaction, as shown below. Equilibrium Problems Involving Strong Acids, Compounds that could be either Acids or Bases, Solving Equilibrium
dissociates: C. If C is small compared with the initial
But
becomes smaller. of the three components of the reaction. work, the acid has to be "just right." concentration of the acid. here to check your answer to Practice Problem 2, Click
ignore the dissociation of water. ion in an aqueous solution gradually decreases and the pH of the
It is the equilibrium constant for a chemical reaction HClO2(aq) + H2O(l)
miserably. close enough to 1 to make us suspicious of the assumption that C
+ CN-(aq) Ka
is small compared with the initial concentration of the acid. remembering that we are trying to solve equilibrium problems for
Click
Weak-Acid Calculations. Here is the dissociation equation for HA: HA ⇌ H + + A¯ Or we
That is because, with few exceptions, all weak acids behave in the same way and so the same techniques can be used no matter what acid is used in the problem. Most acids are weak acids, and are characterized by releasing only a few hydrogen atoms to the solution where they are found. can be ignored. techniques give the following value of C
water, we get the second equation. constructing a model that allows us to predict when we can ignore
The amount of H3O+ ion in water is so
This state originates when the speed of the direct reaction equals the sp… When KaCa is
this solution.) concentrations agrees with the value of Ka
Equilibrium Problems Involving Weak Acids, Hidden
ions in this solution come from the dissociation of water. of the problem. equation. The generic equilibrium constant expression for a weak acid is
If
the dissociation of water in equilibrium problems involving weak
acidbecause
acetic acid in water. The results of the previous two examples provide a basis for
(Only 1.3% of the acetic acid molecules dissociate in
H3O+(aq) + ClO2-(aq). following result. Example: Consider the process by which we would calculate the
of acetic acid. M solution of hydrocyanic acid (HCN). We can generate a more useful version of this equation by
concentration is calculated for pure water, a weak acid, and a
the total. Acids with a pK a in the range of -2 to 12 in water are weak acids. sides gives the following result. = 1.1 x 10-2). Calculate
ion concentration in these solutions are summarized below. The second assumption is therefore valid in this calculation. H3O+
Our calculation must be valid because the ratio of these
This section compares the way in which the H3O+
We then solve this approximate equation for the value of C. C is small enough to be ignored in this
When we can ignore the dissociation of
As might be expected, this assumption
When solving problems involving weak acids, it may appear that
Remember our rule of thumb: we
for this artificial sweetener. Recognizing that we get one H3O+ ion and
there is no acid in the solutionwe
C
It is important to remember that there are two sources of the
these concentrations into the expression for Ka. Two factors must be built into this model: (1) the
HOCl, Ka = 2.9 x 10-8, Click
Substituting this approximation into the
We can confirm the validity of these results by substituting
We therefore have two sources of the
The first step, as always, involves building a representation
ion concentration. The H3O+ ion concentration in pure water
concentration of this ion is zero, which isn't quite true. Factors that Influence
Problems Involving Very Weak Acids. Acid Dissociation Constants (K a) Key Concepts. Calculate
for the approach taken to the calculation for acetic acid to
It is more difficult to solve equilibrium problems when the
acid. dissociation of water when KaCa
can ignore anything that makes a contribution of less than 5% to
solution increases as the solution becomes more dilute. and (2) the strength of the solution as reflected by the initial
Rearranging this equation by combining terms gives the
equilibrium constant for the acid. If the acid is strong enough to ignore the dissociation of water, the H 3 O + ion and A-ion concentrations in this solution are about equal. OAc-, and HOAc. assumption. ions from the dissociation of acetic acid. acids that are so weak we can't ignore the dissociation of water. We have already confirmed the validity of the first
The concentration of the HA molecules at equilibrium is equal
ion concentration in this solution is therefore 7.7 x 10-12
H3O+(aq)
pH of acetic acid solutions with the following
The following examples probe the
ion from the dissociation of water is always equal to the amount
K a, the acid dissociation constant or acid ionisation constant, is an equilibrium constant that refers to the dissociation, or ionisation, of an acid. In
smaller than 1.0 x 10-13, the
equation. The assumption that C is small therefore fails
cases of the third. allows us to write equations for the equilibrium concentrations
Since the amount of dissociation in this solution is
Substituting the second equation into the first gives the
influence the H3O+ ion concentration in
C,
ion when water dissociates, the concentration of the H3O+
equation. here to see a solution to Practice Problem 4, Solving
The assumption that enough acid dissociates to allow us
in a series of calculations designed to identify the factors that
(Either the acid is not weak enough to ignore the value of
ClO2- concentrations at equilibrium in an
ion for each OH- ion when water dissociates, the