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Due to the lone pair of electrons on the highly electronegative N atom,NH 3 molecules will readily attach a free hydrogen ion forming theammonium ion NH 4 + . When we measure the concentration ofOH – for various initial concentration of NH 3 in water, we observe the results in [link] . We should anticipate that a base ionization equilibrium constant might exist comparable to the acidionization equilibrium constant, and in [link] , we have also calculated the value of the function defined as:
| C 0 (M) | [OH – ] | K b | pH |
|---|---|---|---|
| 0.50 | 3.2 × 10 –3 | 2.0 × 10 –5 | 11.5 |
| 0.20 | 2.0 × 10 –3 | 2.0 × 10 –5 | 11.3 |
| 0.10 | 1.4 × 10 –3 | 2.0 × 10 –5 | 11.1 |
| 0.050 | 9.7 × 10 –4 | 1.9 × 10 –5 | 11.0 |
| 0.020 | 6.0 × 10 –4 | 1.9 × 10 –5 | 10.8 |
| 0.010 | 4.2 × 10 –4 | 1.9 × 10 –5 | 10.6 |
| 0.005 | 3.0 × 10 –4 | 1.9 × 10 –5 | 10.5 |
| 0.001 | 1.3 × 10 –4 | 1.8 × 10 –5 > | 10.1 |
| 0.0005 | 8.7 × 10 –5 | 1.8 × 10 –5 | 9.9 |
Given that we have dissolved a base in pure water, we might be surprised to discover the presence of positivehydrogen ions, H 3 O + , in solution, but a measurement of the pH for each of the solutionsreveals small amounts. The pH for each solution is also listed in [link] . The source of these H 3 O + ions must be the autoionization of water. Note, however, that in each case in basic solution, the concentration ofH 3 O + ions is less than that in pure water. Hence, the presence of the base in solution has suppressed the autoionization. Because ofthis, in each case the pH of a basic solution is greater than 7.
Base ionization is therefore quite analogous to acid ionization observed earlier. We now consider a comparisonof the strength of an acid to the strength of a base. To do so, we consider a class of reactions called "neutralizationreactions" which occur when we mix an acid solution with a base solution. Since the acid donates protons and the base acceptsprotons, we might expect, when mixing acid and base, to achieve a solution which is no longer acidic or basic. For example, if we mixtogether equal volumes of 0.1M HCl(aq)and 0.1M NaOH(aq),the following reaction occurs:
The resultant solution is simply a salt solution withNaCl dissolved in water. This solution has neither acidic nor basicproperties, and the pH is 7; hence the acid and base have neutralized each other. In this case, we have mixed together astrong acid with a strong base. Since both are strong and since we mixed equal molar quantities of each, the neutralization reactionis essentially complete.
We next consider mixing together a weak acid solution with a strong base solution, again with equal molarquantities of acid and base. As an example, we mix 100ml of 0.1M acetic acid(HA) solution with 100ml of 0.1M sodium hydroxide. In this discussion,we will abbreviate the acetic acid molecular formula CH 3 COOH asHA and the acetate ionCH 3 COO – as A – . The reaction ofHA andNaOH is:
is the acetate ion in solution, formed when an acetic acid molecule donates the positive hydrogen ion. We have thus created a saltsolution again, in this case of sodium acetate in water. Note that the volume of the combined solution is 200ml, so the concentrationof sodium acetate (NaA)in solution is 0.050M.
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