Acid or base strength refers to the extent to which a chemical dissociates into ions in the presence of water. The logarithmic pH scale serves as an indication of the concentration of dissociated ions, with values ranging from zero to 14. Pure water has a neutral pH of 7. The logarithmic nature of the pH scale indicates that acidity or basicity multiplies by a factor of 10 for each pH interval.
Generally, a strong acid has a pH of about zero to 3. The stronger the acid, the better it dissociates in an aqueous solution, releasing more cationic hydrogen (H+) ions. Examples of strong acids include hydrochloric acid (HCl), hydrobromic acid (HBr), perchloric acid (HClO4), and sulfuric acid (H2SO4). However, because pH measures the amount of hydrogen ions released in a solution, even a very strong acid can have a high pH reading if its concentration is very dilute. For instance, a 0.0000001 molar HCl solution has a pH of 6.79. As a strong acid, HCl exhibits 100 percent dissociation, but the extremely low concentration of hydrogen ions it releases in this case gives it a nearly neutral pH.
A weak acid, on the other hand, fails to ionize completely. It releases fairly low concentrations of hydrogen ions in an aqueous solution, resulting in a pH range of about 5 to just below 7. Examples include acetic acid (CH3COOH), the main component of vinegar, and formic acid (HCOOH), the acid responsible for the sting of ant bites. Again, there are exceptions to this general pH range. A sufficiently concentrated weak acid can still produce a low pH readout. A 1.0 molar CH3COOH solution, for example, has a pH of 2.37.
Like strong acids, a strong base dissociates nearly completely in water; however, it releases hydroxide (OH-) ions rather than H+. Strong bases have very high pH values, usually about 12 to 14. Well-known examples of strong bases include caustic soda or sodium hydroxide (NaOH), as well as lye or potassium hydroxide (KOH). Hydroxides of alkali or Group 1 metals are generally strong bases.
The pH of a weak base falls somewhere between 7 and 10. Like weak acids, weak bases do not undergo complete dissociation; instead, their ionization is a two-way reaction with a definite equilibrium point. While strong bases release hydroxide ions via dissociation, weak bases generate hydroxide ions by reacting with water. Ammonia (NH3) and methylamine (CH3NH2) are examples of weak bases.
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