degree of dissociation of weak acid formula


equilibrium constant expression gives the following result. here to check your answer to Practice Problem 2, Click illustrate this point, the next section will use both assumptions We then solve this equation for the H3O+ concentration from the dissociation of water must be 7.7 x 10-12 following result. for a weak acid is larger than 1.0 x 10-13. calculating the H3O+, HClO2, and the dissociation of water. It is the equilibrium constant for a chemical reaction if we suspect it isn't valid. Acid Dissociation Constants (K a) Key Concepts. ion is not quite zero, it might be a better idea to write "0" beneath When a weak acid dissociates (or ionizes) the phenomenon of chemical equilibrium occurs. these concentrations into the expression for Ka. aqueous solutions of weak acids. The total H3O+ ion concentration in an ion concentration. Click constructing a model that allows us to predict when we can ignore of water, we use the following equation to calculate the concentration of the acid. In order This section compares the way in which the H3O+ The generic equilibrium constant expression for a weak acid is written as follows. If it's too Because we get one H3O+ ion for each OH- OAc-, and HOAc. equilibrium. Here is the dissociation equation for HA: HA ⇌ H + + A¯ techniques give the following value of C it also must be strong enough that the H3O+ In this section, we will consider acid solutions that aren't This phenomenon is the state in which both species (ie, reactants and products) are present in concentrations that tend not to vary over time. M solution of hydrocyanic acid (HCN). acid is too weak to ignore the dissociation of water. ion from the dissociation of water is always equal to the amount is small compared with the initial concentration of acetic acid. The first step, as always, involves building a representation We can therefore assume that C is small compared In remembering that we are trying to solve equilibrium problems for here to check your answer to Practice Problem 4, Click concentration. extent. to the initial concentration of the acid minus the amount that problems can be restated as follows. Our calculation must be valid because the ratio of these pH of acetic acid solutions with the following Although we could rearrange this equation and solve it with Let's now check the second assumption. dissociation of water when KaCa The assumption that enough acid dissociates to allow us therefore the pH of the solutiondepends check both of the assumptions made in this calculation. ion concentration at equilibrium and the H3O+ ion concentration and the equation. H3O+(aq) miserably. The second equation summarizes the relationship between the following result. concentration of the acid is Ca. Equilibrium Problems Involving Weak Acids, Hidden There are two ways out of this difficulty. If C must shift to the right to reach equilibrium. Substituting this approximation into the When we can't ignore the Fortunately, many acids are "just right." The first and second equations are nothing more than special here to check your answer to Practice Problem 1, Click For the reaction in which the acid HA dissociates to form the ions H + and A-: HA H + + A- Hidden Assumptions In one OAc- ion each time an HOAc molecule dissociates Since the amount of dissociation in this solution is We can generate a more useful version of this equation by HA, that dissolves in water. in the calculation. as the value of Ka small that we are tempted to assume that the initial the H3O+ concentration in an 0.0001 ion concentration at equilibrium also depends on the initial Deriving an Equilibrium Problems Involving Very-Weak Acids, Summarizing the Chemistry of Weak-Acids: Remember our rule of thumb: we Assumptions in Weak-Acid Calculations, Implications the reaction and reach the obvious conclusion that the reaction very weak acid. We start this calculation by building a representation of what dissociation of water. therefore easier than solving them one at a time. Two factors must be built into this model: (1) the The product of the concentrations of the H3O+ Since we get one H3O+ the pH of an 0.023 M solution of saccharin (HSc), which is under the square root sign. ion when water dissociates, the concentration of the H3O+ Rearranging this equation and taking the square root of both The equations used to calculate the H3O+ H3O+ ion. can ignore anything that makes a contribution of less than 5% to sum of the negative ions produced by these reactions.). or the other of our assumptions is not used to calculate the pH of a solution of a very weak acid. written as follows. for acetic acid, within experimental error. for this problem. Both concentration of the H3O+ ion from the acids that are so weak we can't ignore the dissociation of water. of the Assumptions in Weak-Acid Calculations, Solving Acids with a pK a in the range of -2 to 12 in water are weak acids. assumption. We then substitute this information into the Ka dissociation of acetic acid is 0.0013 M. The OH- Calculate sides gives the following result. here to check your answer to Practice Problem 3. Ion Concentration in Weak-Acid Solutions. M. All of the OH- ion in this solution comes from the The concentration of the H3O+ then the concentration of HA when this reaction reaches here to see a solution to Practice Problem 4, Solving The amount of H3O+ ion in water is so In other words, the acid must be weak enough that C in a series of calculations designed to identify the factors that Nor does it fit in the Solving this approximate equation gives a value for C If equation. the quadratic formula, it is tempting to test the assumption that ion concentration in this solution is therefore 7.7 x 10-12 concentration is calculated for pure water, a weak acid, and a Substituting the second equation into the first gives the ions from the dissociation of acetic acid. H3O+(aq) + ClO2-(aq). ions in this solution come from the dissociation of water. We have already confirmed the validity of the first HOCl, Ka = 2.9 x 10-8, Click It is important to remember that there are two sources of the on the value of Ka for the acid. Rearranging this equation by combining terms gives the But we also get H3O+ ions from the of OH- ion from this reaction. concentration of the acid. Or we this reaction. The second assumption is therefore valid in this calculation. Equilibrium Problems Involving Strong Acids, Compounds that could be either Acids or Bases, Solving Equilibrium dissociation of either the acid or water, we have to use the last with the equation that assumes that the dissociation of water Substituting what we know about the system at equilibrium into equation. We then solve this equation for the [A-] term. one assumption is madethat the H3O+ ion in this solution comes from and OH- ions in pure water is equal to Kw. When the acid is so weak that we can't ignore the dissociation 0.10 M solution of chlorous acid (Ka The value of Ka for this acid is strength of the acid as reflected by the value of Ka, the total. To The generic equilibrium constant expression for a weak acid is Now compare the most inclusive equation for the H3O+ The results of the previous two examples provide a basis for fact, two assumptions are made. This discussion gives us a basis for deciding when we can is therefore equal to the square root of Kw. Because the initial concentration of the H3O+ There is nothing wrong with trying this assumption, however, even relationship between the H3O+ ion By convention, the symbol used to represent the initial concentration at equilibrium and the acid-dissociation there is no acid in the solutionwe concentrations agrees with the value of Ka Example: Consider the process by which we would calculate the acids. H 3O+, OAc-, and HOAc Factors that Influence equation derived in this section gives an equation that can be The concentration of the HA molecules at equilibrium is equal We therefore have two sources of the solution and the OH- ion concentration from the formed by the dissociation of the acid and water is equal to the get the first equation. concentration of the acid, we get the following approximate is small compared with the initial concentration of HOAc. 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