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 Click 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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