A simple buffer solution contains a weak acid (such as acetic acid) plus its conjugate base (often in the form of a salt such as sodium acetate). Or, wording it differently, you can have a weak base plus its conjugate acid in the same solution. However you say it, you must have a conjugate pair in the same solution for it to be a buffer solution.
Things to check to tell if it's a buffer solution:
1. Is the acid a weak acid?
2. Is the conjugate base of that weak acid present also? Remove one H+ from the weak acid to get the formula of the conjugate base.
Or, from the weak base standpoint,
1. Is a weak base present?
2. Is the conjugate acid of that weak base present also? Add one H+ to the weak base to get the formula of the conjugate acid.
Remember to ignore spectator ions which have no effect on pH, such as chloride ion and sodium ion. NH4Cl is a weak acid because of the ammonium ion, NH4+. The Cl�?/SUP> ion is the conjugate base of the strong monoprotic acid HCl, so it is such a weak base that it is neutral, or a "spectator" as far as pH is concerned. (The stronger the acid, the weaker its conjugate base will be.)
This is what you have, NH3, which is a weak base, and its conjugate acid, NH4+, in the same solution. Therefore, this will be a buffer solution. 
Steve
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