Hydronium ions equilibrium concentrations

Because a proton transfer equilibrium is established as soon as a weak acid is dissolved in water, the concentrations of acid, hydronium ion, and conjugate base of the acid must always satisfy the acidity constant of the acid. We can calculate any of these quantities by setting up an equilibrium table like that in Toolbox 9.1. 

Proton transfer equilibrium is established as soon as a weak base is dissolved in water, and so we can calculate the hydroxide ion concentration from the initial concentration of the base and the value of its basicity constant. Because the hydroxide ions are in equilibrium with the hydronium ions, we can use the pOH and pKw to calculate the pH. 

Step 5 Use an equilibrium table to find the H.O concentration in a weak acid or the OH concentration in a weak base. Alternatively, if the concentrations of conjugate acid and base calculated in step 4 are both large relative to the concentration of hydronium ions, use them in the expression for /<, or the Henderson—Hasselbalch equation to determine the pH. In each case, if the pH is less than 6 or greater than 8, assume that the autoprotolysis of water does not significantly affect the pH. If necessary, convert between Ka and Kh by using Kw = KA X Kb. 

Sulfides with widely different solubilities and solubility products can be selectively precipitated by adding S2 ions to the solution removed from the chlorides in the first step (see Fig. 11.20). Some metal sulfides (such as CuS, HgS, and Sb2S3) have extremely small solubility products and precipitate if there is the merest trace of S2" ions in the solution. Such a very low concentration of S2 ions is achieved by adding hydrogen sulfide, H2S, to an acidified solution. A higher hydronium ion concentration shifts the equilibrium... 

To verify the presence of Zn2+ ions in the solution remaining after the first two steps, we add H2S followed by ammonia. The base removes the hydronium ion from the H2S equilibrium, which shifts the equilibrium in favor of S2- ions. The higher concentration of S2 ions increases the Qsp values of any remaining metal sulfides, such as ZnS or MnS, above their fCsp values, and they precipitate. 

C16-0007. Write the equilibrium constant expression for HCl gas dissolving in water to produce hydronium ions, and indicate the appropriate concentration units for each ... 

The equilibrium concentration of hydronium ions is provided, which suggests that one of the products is H3 O. Benzoic acid is a weak acid. Thus the correct chemical reaction is proton transfer from benzoic acid to water ... 

Construct a table of initial concentrations, changes in concentration, and equilibrium concentrations for each species that appears in the equilibrium constant expression. The equilibrium concentrations from the last row of the table are needed to find Kgq. Start by entering the data given in the problem. The initial concentration of benzoic acid is 0.125 M. Pure water contains no benzoate ions and a negligible concentration of hydronium ions. The problem also states the equilibrium concentration of hydronium ions, 0.0028 M. 

The concentration of hydronium ion at equilibrium is in the 10 range, so a value of ICsq in the 10 range is reasonable. 

Calculate the equilibrium concentrations of acetic acid, acetate ion, and hydronium ion in a 2.5 M solution of acetic acid. 

The water equilibrium describes an inverse relationship between [H3 0+] eq nd [OH-]gq. When an acid dissolves in water, the hydronium ion concentration increases, so the hydroxide ion concentration must decrease to maintain the product of the concentrations at 1.0 X 10. Similarly, the hydroxide ion concentration increases when a base dissolves in water, so the hydronium ion concentration must decrease. 

This aqueous solution is 5.0 x 10 M HCIO4, so [ H3 O ] = [CIO4 ] = 5.0 x 10 M. The final concentrations are found using an equilibrium analysis. Set up a concentration table for the water dissociation equilibrium, and define the change in hydronium ion concentration as x ... 

In any solution of an acid, the total hydronium and hydroxide ion concentrations include the 10" M contribution from the water reaction. This example illustrates, however, that the change in hydronium ion concentration due specifically to the water equilibrium is negligibly small in an aqueous solution of a strong acid. This is true for any strong acid whose concentration is greater than 10 M. Consequently, the hydronium ion concentration equals... 

For this example, we summarize the first four steps of the method The problem asks for the concentration of ions. Sodium hydroxide is a strong base that dissolves in water to generate Na cations and OH- anions quantitatively. The concentration of hydroxide ion equals the concentration of the base. The water equilibrium links the concentrations of OH" and H3 O" ", so an equilibrium calculation is required to determine the concentration of hydronium ion. What remains is to organize the data, carry out the calculations, and check for reasonableness. 

A logarithmic scale is useful not only for expressing hydronium ion concentrations, but also for expressing hydroxide ion concentrations and equilibrium constants. That is, the pH definition can be generalized to other quantities pOH = - log [OH ] p Tg = - log Tg p log... 

To find the pH of a solution, first compute [H3 O ] or [OH ] and then apply Equation or. Sodium hydroxide is a strong base, and the water equilibrium provides the link between hydroxide and hydronium ion concentrations. 

Most acids and bases are weak. A solution of a weak acid contains the acid and water as major species, and a solution of a weak base contains the base and water as major species. Proton-transfer equilibria determine the concentrations of hydronium ions and hydroxide ions in these solutions. To determine the concentrations at equilibrium, we must apply the general equilibrium strategy to these types of solutions. 

Measurements of the pH of solutions of acids show that, except for strong acids, the hydronium ion concentration is smaller than would be expected if proton transfer were quantitative. An acid that reaches equilibrium when only a small fraction of its molecules has transferred protons to water is called a weak acid. One example is benzoic acid, treated in Example. A 0.125 M solution of this acid has [H3 ] = 0.0028 M (pH = 2.55). As... 

The equilibrium concentration of hydronium ions is in the 10 M range, so this value for appears reasonable. Notice that the problem identified this solution as 0.25 M HF, but at equilibrium, [HF] = 0.24 M. Conventionally, a solution s concentration is stated as its initial concentration, even though the equilibrium concentrations may differ slightly from this initial concentration. 

To determine percent ionization, we need to know the equilibrium concentration of hydronium ions. This requires an equilibrium calculation, for which we follow the seven-step method. We need to set up the appropriate equilibrium expression and solve for [H3 O, after which we can use Equation to... 

The equilibrium concentration of hydronium ions is smaller than the starting concentration of HF, as it must be. We use two significant figures to match the precision in the Ag value. 

After completing our analysis of the effects of the dominant equilibrium, we may need to consider the effects of other equilibria. The calculation of [H3 O ] in a solution of weak base illustrates circumstances where this secondary consideration is necessary. Here, the dominant equilibrium does not include the species, H3 O, whose concentration we wish to know. In such cases, we must turn to an equilibrium expression that has the species of interest as a product. The reactants should be species that are involved in the dominant equilibrium, because the concentrations of these species are determined by the dominant equilibrium. We can use these concentrations as the initial concentrations for our calculations based on secondary equilibria. Look again at Example for another application of this idea. In that example, the dominant equilibrium is the reaction between hypochlorite anions and water molecules H2 0 l) + OCr(c2 q) HOCl((2 q) + OH ((2 q) Working with this equilibrium, we can determine the concentrations of OCl, HOCl, and OH. To find the concentration of hydronium ions, however, we must invoke a second equilibrium, the water equilibrium 2 H2 0(/) H3 O (a q) + OH (a q)... 

The low concentration of an indicator also explains why the presence of this weak acid does not change the pH of the solution. Indicators are always present as minor species in solution, never as major species. Thus, the dominant equilibrium that determines the pH of a solution never involves the indicator. The of the substance being titrated establishes the equilibrium concentration of hydronium ions. This, in turn, establishes whether the... 

EXAMPLE 20.7. Calculate the hydronium ion concentration of a 0.200 M solution of acetic acid, using the equilibrium constant of Example 20.6. 

At 25°C, only about two water molecules in one billion dissociate. This is why pure water is such a poor conductor of electricity. In neutral water, at 25°C, the concentration of hydronium ions is the same as the concentration of hydroxide ions 1.0 x 10 mol/L. These concentrations must be the same because the dissociation of water produces equal numbers of hydronium and hydroxide ions. Because this is an equilibrium reaction, and because the position of equilibrium of all reactions changes with temperature, [HaO" ] is not 1.0 x 10 mol/L at other temperatures. The same is true of [OH ]. 

So few ions form that the concentration of water is essentially constant. The product fclHaO] is equal to the product of the concentrations of hydronium ions and hydroxide ions. The equilibrium value of the concentration ion product [HaO HOH"] at 25°C is called the ion product constant for water. It is given the symbol K . 

In water, the hydronium ion concentration arises by the self-dissociation equilibrium (see Section 4.4) ... 

Weak acids are not completely ionized in aqueous solutions, and the amount of ionization, and thus hydronium ion concentration, is governed by the equilibrium... 

To predict the pH of mixtures of weak acids or bases and their salts quantitatively, we set up an equilibrium table, as described in Toolbox 10.1. Then we use the acidity or basicity constant to calculate the concentration of hydronium ions present in the solution. The only difference is that now the conjugate acid and base are both present initially, so the first line of the table must have their initial concentrations. For instance, in the mixed acetic acid/sodium acetate solution, both acetic acid and its conjugate base, acetate ions, are present initially. In the ammonia/ammo-nium chloride solution, both the base (ammonia) and its conjugate acid (the ammonium ions) are present initially. 

Use the Equilibrium Constant for Acids activity (eChapter 15.9) to experiment with the dissociation of a weak acid. What would be the hydronium ion concentration and pH of 0.10 M solutions of weak acids with Ka = 1 X 10"5 Ka= IX 10"10 and Ka = IX 10"15 Describe the relationship between the pH and the strength of an acid for acids at equal concentrations. 

This further reduces the hydronium ion concentration, and this effect is even greater at elevated temperatures because the equilibrium constant decreases. 

Point A on the titration curve is half-way to the first equivalence point with 0.5 mol of base added per mole of phosphoric acid. If the pH at this point were between pH > 5 and pH < 9, die concentration of H3P04 and of H2P04 ion could be assumed to be equal. However, because the pH is approximately pH 3, the concentration of H3P04 at this point is less than that of the H2P04 ion by an amount equivalent to about twice the hydronium ion concentration. Thus, the correct expressions at the half-equivalence point for the equilibrium concentrations are... 

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