Hydronium ions calculating concentration

Hydronium ion, 187 concentration calculation, 192 concentration and pH, 190 model, 186 Hydroquinone, 345 Hydrosphere, 437 composition, 439 Hydroxide ion, 106, 180 Hydroxides of lhird row, 371 Hydroxylamine, 251 Hydroxyl group, 329 Hypobromiie ion, 422 Hypochlorite ion, 361 Hypochlorous acid, structure, 359 Hypophosphorous acid, 372 Hypothesis, Avogadro s, 25, 52... 

EXAMPLE 10.4 Sample exercise Calculating the hydronium ion concentration from the pH... 

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. 

STRATEGY Calculate the hydronium ion concentration from the pH and then calculate the value of /C, from the initial concentration of the acid and the hydronium ion concentration. 

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. 

Suppose we were asked to estimate the pH of 1.0 X 10 x m HCl(aq). If we used the techniques of Example 10.3 to calculate the pH from the concentration of the acid itself, we would find pH = 8.00. That value, though, is absurd, because it lies on the basic side of neutrality, whereas HC1 is an acid The error stems from there being two sources of hydronium ions, whereas we have considered only one. At very low acid concentrations, the supply of hydronium ions from the autoprotolysis of water is close to the supply provided by the very low concentration of HC1, and both supplies must be taken into account. The following two sections explain how to take autoprotolysis into account, first for strong acids and bases and then for weak ones. 

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. 

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

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. 

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... 

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)... 

At the beginning of the titration, the diprotic acid (represented by H2 A) and H2 O are the only major species in the solution. As we describe in Chapter 17, the hydronium ion concentration can be calculated from the... 

Atmospheric O2 has a partial pressure of 0.20 bar, and atmospheric water vapor is saturated with carbon dioxide. This dissolved CO2 forms carbonic acid, which generates a hydronium ion concentration of about 2.0 X 10 M. The Nemst equation allows calculation of the half-cell potential for the reduction of 02(g) under these... 

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. 

EXAMPLE 20.8. Calculate the hydronium ion concentration in pure water at 250C. 

Buffers are solutions that resist a change in pH when we add an acid or base. A buffer contains both a weak acid (HA) and its conjugate base (A-). The acid part will neutralize any base added and the base part of the buffer will neutralize any acid added to the solution. We may calculate the hydronium ion concentration of a buffer by rearranging the Ka expression to yield the Henderson-Hasselbalch equation, which we can use to calculate the pH of a buffer ... 

Calculate the concentration of hydronium ions in each solution. 

If the pH is known, then we can calculate the hydronium ion concentration. Since... 

Note again that brackets are used to represent molar concentrations, meaning [H30+] is read the molar concentration of hydronium ions. For understanding the logarithm function, see the Calculation Corner on page 344. 

Consider a neutral solution that has a hydronium ion concentration of 1.0 X 10"7 M. To find the pH of this solution, we first take the logarithm of this value, which is —7 (see the Calculation Corner on logarithms). The pH is, by definition, the negative of this value, which means —(—7) = +7. Hence, in a neutral solution, where the hydronium ion concentration equals 1.0 X 10-7 M, the pH is 7. 

When the pH of a solution is 1, the concentration of hydronium ions is 10-1 M= 0.1 M. Assume that the volume of this solution is 500 mL and that the solution is not buffered. What is the pH after 500 mL of pure water is added You will need a calculator with a logarithm function to answer this question. 

The concentration of hydronium ions in the pH = 1 solution is 0.1 M. Doubling the volume of solution with pure water means that its concentration is cut in half. The new concentration of hydronium ions after the addition of 500 mL of water, therefore, is 0.05 M. To calculate for pH ... 

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