What does a solution that buffers against both an acid or base contain?

ex:





an unknown solution that buffers against acids contains a OH group


an unknown solution that buffers against bases contains a H group





What does a solution that buffers against both an acid or base contain?|||It contains a weak acid and its conjugate base.





The conjugate base of any acid is obtained by removing a H+ from the acid.





So, HCl --%26gt; Cl-, HNO3 --%26gt; NO3-, and so on.





Weak bases that are conjugate bases of weak acids are stronger bases than weak bases that are conjugate bases of strong acids. So when you have significant amount of a weak acid and its conjugate base in solution, a buffer is formed.





Example: NH3 and NH4+. The equilibrium in this buffer is:





NH4+ %26lt;----%26gt; NH3 + H+. Ka = 5.6 X 10-10, pka = 9.25.





when a strong acid is added to the solution, it reacts with the NH3 in the solution, NH3 is a stronger base than water, so it prevents the solution from building up the concentration of hydronium in it. Likewise, is a strong base is added to the buffer, it react with NH4+ before it does react with water, preventing the build up of hydroxide ions in the solution.





This will continue as long as there is enough weak acid and its conjugate base in solution. If the ratio of acid/base surpasses 10 or falls below 0.1, the buffer starts to become ineffective. That is why is recommended that to control a pH, you must choose a buffer as close to the pH that you wish to control. So the NH4+/NH3 buffer is good from around 8.25 to 10.25, optimal at 9.25, but if you go below 8.25 or above 10.25, the buffer is not that good.





Hope it helps.|||A buffer would contain a weak acid and one of its salts or a weak base and one of its salts





eg. ethanoic acid and any ethanoate


or aqueous ammonia and an ammonium salt