# バッファ容量とは何ですか？バッファはどのように「使い切っていますか？」

I'm a little confused on something: A buffer has a certain capacity and then runs out.
But, if the buffer is given by: \$\$\ce{A- + H2O <=> HA + OH-},\$\$ when the \$\ce{A-}\$ reacts with the water, the reaction shifts to the left. So how can it "run out"? If \$Q\$ just keeps becoming \$0\$, shouldn't the reaction keep shifting to maintain \$K\$? Does the right side, perhaps, approach \$0\$ as it keeps decreasing in concentration to maintain \$K\$, but it can never become \$0\$ molar, because that wouldn't maintain \$K\$?
I guess if \$\ce{H2O}\$ was "aqueous"/had a concentration and was therefore used in the \$K\$ expression, this the \$K\$ would always be maintained.
But how does the buffer "run out" of \$\ce{A-}\$ to react when \$\ce{H2O}\$ is liquid (not aqueous)? It doesn't make sense as wouldn't the reaction always adjust to make more \$\ce{A-}\$ to maintain \$K\$?

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## 1 答え

In the equation you wrote, \$\$\ce{A- + H2O <=> HA + OH-},\$\$ think about the effect of adding acid. Acid reacts with the \$\ce{OH-}\$, producing water and removing \$\ce{A-}\$ from solution. The \$\ce{OH-}\$ and \$\ce{A-}\$ cannot be removed from solution but their concentrations can be driven to negligible levels by the addition of sufficient quantities of acid.

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