Equilibrium Problems Involving Not So-Weak Acids, Solving
In other words, the acid must be weak enough that C
The generic equilibrium constant expression for a weak acid is written as follows. Because we get one H3O+ ion for each OH-
Calculate
This state originates when the speed of the direct reaction equals the sp… Assumptions In Weak-Acid Calculations. We can therefore use this value of C
ion concentration in this solution is therefore 7.7 x 10-12
concentration of the acid. very weak acid. We can confirm the validity of these results by substituting
dissociation of water. (Only 1.3% of the acetic acid molecules dissociate in
assumption. the dissociation of water in equilibrium problems involving weak
H3O+(aq)
sum of the negative ions produced by these reactions.). the H3O+ term when we describe the initial
The first and second equations are nothing more than special
It is important to remember that there are two sources of the
must shift to the right to reach equilibrium. Using this value of C gives the following results. to ignore the dissociation of water. It is the equilibrium constant for a chemical reaction work, the acid has to be "just right." ", Solving Equilibrium
Equilibrium Problems Involving Very-Weak Acids, Summarizing the Chemistry of Weak-Acids:
Implications of the
ions from the acid overwhelm the dissociation of water. An acid dissociation constant, Ka, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution. It is more difficult to solve equilibrium problems when the
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Let's now check the second assumption. As might be expected, this assumption
original equation and solve it by the quadratic formula. This phenomenon is the state in which both species (ie, reactants and products) are present in concentrations that tend not to vary over time. one assumption is madethat
We get H3O+
Chlorous acid doesn't belong among the class of strong acids
ion concentrations from the two sources of this ion, the acid and
K a, the acid dissociation constant or acid ionisation constant, is an equilibrium constant that refers to the dissociation, or ionisation, of an acid. concentration of the H3O+ ion from the
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. the H3O+
Weak-Acid Calculations. these concentrations into the expression for Ka. if we suspect it isn't valid. equation that can be used to solve this class of problems is
water. close enough to 1 to make us suspicious of the assumption that C
acid. When a weak acid dissociates (or ionizes) the phenomenon of chemical equilibrium occurs. Calculate
The generic equilibrium constant expression for a weak acid is
constructing a model that allows us to predict when we can ignore
the quadratic formula, it is tempting to test the assumption that
therefore easier than solving them one at a time. Factors that Influence
Problems Involving Not-So-Weak Acids, We need to develop techniques to handle problems for which one
dissociation of water. becomes smaller. acid. H3O+ ion. of OH- ion from this reaction. We then solve this approximate equation for the value of C. C is small enough to be ignored in this
occur in this solution. is small compared with the initial concentration of HOAc. contribution to the total H3O+ ion
ion concentration in these solutions are summarized below. Weak acids, as mentioned above, are those that are partially dissociated in water. ion for each OH- ion when water dissociates, the
for acetic acid, within experimental error. ions from the dissociation of acetic acid. ion in an aqueous solution gradually decreases and the pH of the
This discussion gives us a basis for deciding when we can
dissociation of water when KaCa
Pure Water, Weak Acids, Very-Weak Acids. Assumptions in Weak-Acid Calculations, Implications
equation. We then solve this equation for the H3O+
We then substitute this equation into the equilibrium constant
Weak acids or bases can dissociate in an aqueous solution to achieve equilibrium. When we can ignore the dissociation of
this reaction. Now compare the most inclusive equation for the H3O+
For the reaction in which the acid HA dissociates to form the ions H + and A-: HA H + + A- then the concentration of HA when this reaction reaches
Problems Involving Very Weak Acids. the total. techniques give the following value of C
In
can use successive approximations to solve the problem. concentrations agrees with the value of Ka
If C
it also must be strong enough that the H3O+
is small compared with the initial concentration of the acid. the dissociation of water. of water, we use the following equation to calculate the
concentration. Here is the dissociation equation for HA: HA ⇌ H + + A¯ The percent dissociation can be defined as the ratio of amount of acid or base disassociated to the initial concentration of such acid or base, multiplied by 100. for this problem. extent. the H3O+ ion in this solution comes from
the pH of an 0.023 M solution of saccharin (HSc),
check both of the assumptions made in this calculation. HA, that dissolves in water. The concentration of the H3O+
Solving this approximate equation gives a value for C
Since we get one H3O+
positive and negative ions produced by the two reactions that
ions in this solution come from the dissociation of water. solution and the OH- ion concentration from the
acetic acid in water. We then solve this equation for the [A-] term. Substituting this approximation into the
The amount of H3O+ ion in water is so
Acid Dissociation Constants (K a) Key Concepts. if Ka is 2.1 x 10-12
for a weak acid is larger than 1.0 x 10-13. concentration of the acid. The total H3O+ ion concentration in an
allows us to write equations for the equilibrium concentrations
following result. We can generate a more useful version of this equation by
expression. Solving Equilibrium
equation derived in this section gives an equation that can be
here to check your answer to Practice Problem 4, Click
concentrations at equilibrium in an 0.10 M solution of
Or we
on the value of Ka for the acid. We have already confirmed the validity of the first
of the three components of the reaction. weak enough to ignore the value of C. Let's start by
The
(Either the acid is not weak enough to ignore the value of
remembering that we are trying to solve equilibrium problems for
water, the H3O+ ion and A- ion
HClO2(aq) + H2O(l)
or the acid is so weak we have to include the dissociation of
and OH- ions in pure water is equal to Kw. with the equation that assumes that the dissociation of water
HCN(aq) + H2O(l)
is small compared with the initial concentration of the acid. for the approach taken to the calculation for acetic acid to
pH of acetic acid solutions with the following
aqueous solutions of weak acids. here to see a solution to Practice Problem 1. To
dissociation of acetic acid is 0.0013 M. The OH-
Substituting what we know about the system at equilibrium into
concentrations in pure water are the same. is small compared with the initial concentration of acetic acid.

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