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We consider a final note on the autoionization of water. The pH of pure water is 7 at 25°C. Adding any acidto pure water, no matter how weak the acid, must increase [H 3 O + ],thus producing a pH below 7. As such, we can conclude that, for all acid solutions, pH is less than 7, or on the other hand, anysolution with pH less than 7 is acidic.
We have not yet examined the behavior of base molecules in solution, nor have we compared the relative strengthsof bases. We have defined a base molecule as one which accepts a positive hydrogen ion from another molecule. One of the most commonexamples is ammonia, NH 3 . When ammonia is dissolved in aqueous solution, the followingreaction occurs:
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.
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