Answer:
Weak acids and weak bases are only partially dissociated.
Explanation:
When a small amount of an acid or a base is added to water (or a solution of some strong acid or base of some certain [tex]pH \![/tex],) the [tex]pH[/tex] of that solution tends to change significantly.
In contrast, when the same amount of acid or base is added to a buffer system of some certain [tex]pH\![/tex], the [tex]pH[/tex] of this system tends to change by a much smaller amount.
Weak acids and weak bases dissociate only partially. That allows the buffer solutions to contain a large reservoir of both the (undissociated) weak acid (or weak base) and the corresponding conjugate ion, while maintaining the required [tex]pH[/tex].
This reservoir of the partially-dissociated weak acid (or weak base) and its conjugate ion allows the buffer system to absorb [tex]\rm H^{+}[/tex] and [tex]\rm OH^{-}[/tex] that were added to the solution without much change to the [tex]pH[/tex].
For example, consider a [tex]1\; \rm L[/tex] buffer solution that initially included [tex]0.1\; \rm mol[/tex] of acetic acid [tex]\rm CH_3COOH[/tex] (a weak acid with [tex]pK_\text{a} = 4.765[/tex]) and [tex]0.1\; \rm mol\![/tex] of sodium acetate [tex]\rm CH_3COONa[/tex]. The [tex]pH[/tex] of this solution would be [tex]4.765[/tex].
In contrast, if a solution of [tex]\rm HCl[/tex] (a strong acid) and an equal amount of [tex]\rm NaCl[/tex] also needs to be [tex]4.765[/tex], only [tex]10^{-4.765}\; \rm mol \approx 0.0000172\; \rm mol[/tex] of each species could be included.
Assume that [tex]0.001\; \rm mol[/tex] of [tex]{\rm NaOH}\, (s)[/tex] is added to the buffer solution of [tex]\rm CH_3COOH[/tex] and [tex]\rm CH_3COONa[/tex]. The quantity of the partially-dissociated weak acid in this solution is much larger than that of the [tex]\rm OH^{-}[/tex] added to the solution.
Only a small portion of the weak base would be consumed.The [tex]pH[/tex] of the solution would likely change by less than [tex]0.001[/tex].
In contrast, assume that [tex]0.001\; \rm mol[/tex] of [tex]{\rm NaOH}\, (s)[/tex] is added to the [tex]1\; \rm L[/tex] solution with [tex]0.0000172\; \rm mol[/tex] of [tex]\rm HCl[/tex] (fully-dissociated) and [tex]\rm NaCl[/tex].
All the [tex]\rm HCl\![/tex] in this solution would be consumed. The [tex]pH[/tex] of this solution would go well above [tex]7[/tex].
Answer:
Weak acids and weak bases are partially dissociate
Explanation:
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