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Acids and Bases

By Levi Clancy for Student Reader on

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With strong acids and bases, they dissociate completely and you do not have to worry about a pKa value. With weak acids and bases, they do not dissociate completely -- their disassociation (their relative strength) is according to their pKa value (for acids) or pKb value (for bases). A strong acid has a negative pKa and a weak acid has a low positive pKa (higher values are even weak bases). A buffer is a solution of weak acid (or base) and its conjugate base (or acid) that resists changes in pH. Buffer capacity is a buffer's ability to buffer a solution is best when pH = pKa and still useful within ±1 units of the pH. All weak acids have 80% of their deprotonation within ±1 pH of pKa. All have the same titration curve just different pKas. Buffer region is that area when the solution is with ±1 pH units of the pKa. At the pKa or pKb value, the ratio of protonated to deprotonated is equal, it is 1:1. Also, within ±1 pH of the pKa value, the weak acid or base is present in both forms and can thus buffer the solution. The so-called buffer capacity, the ability for acid or base to be absorbed into the solution without drastic changes in pH, is best within a solution within ±1 pH of the pKa value. Zwitterion has neutral charge overall. It can have a negative charge on one atom, so long as a positive charge on another atom can balance it out. Some weak acids are polyprotic and have more than one pKa value, for example one functional group has a pKa1 and another functional group has a pKa2. Eighty percent of acid dissociation occurs within one unit of the pKa, a close range.

pH = - log [H+]

pOH = - log [OH-]

Ka = [H+][A-]/[HA]
A higher pKa will mean a higher proton affinity and a weaker acidity. Conversely, a high ka will mean a lower proton affinity and a higher acidity.

pH = pKa + log [A-]/[HA]

Keq is the tendency for the acid to dissociate, to lose its proton. A higher Keq refers to a stronger acid.

Keq = ka = [H+] [A-] / [HA]

pH = pKa when .5 units of a strong base have been added to one unit of a weak acid. If ph = pKa then [A-] = [HA] and therefore half the acid must be deprotonated. Thus, exactly half of the acid has been deprotonated. Changes in pH will be resisted at this point, and thus this is called a buffer.

Seven standard strong acids
  • Hydrochloric acid: HCl

  • Hydrobromic acid: HBr

  • Hydroiodic acid: HI

  • Sulfuric acid: H2SO4

  • Nitric acid: HNO3

  • Perchloric acid: HClO4

  • Chloric acid: HClO3

Eight standard strong bases
  • Lithium hydroxide: LiOH

  • Sodium hydroxide: NaOH

  • Potassium hydroxide: KOH

  • Rubidium hydroxide: RbOH

  • Calcium hydroxide: Ca(OH)2

  • Strontium hydroxide: Sr(OH)2

  • Barium hydroxide: Ba(OH)2