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Molarity of the water.
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どういった時にでた問題なのかが判らないので，どんな回答をしてよいやら困っているのですが・・・。 モル濃度（molarity）は「溶液１Ｌ中に含まれる溶質のモル数」ですので，純水（水）のモル濃度というのはおかしいのですが，お書きの様に，水１Ｌのモル数は 1000/18 ≒ 55.5 [mol] です。 さて，お書きの例で 1 mM の酢酸溶液を考えてみます。この溶液１Ｌ中には酢酸分子が 1 mmol 存在します。つまり，純水１Ｌと比べると，幾つかの水分子が酢酸分子に置き代わっています。 酢酸１分子の体積が水何分子分か解りませんので，1 mM 酢酸溶液中の水分子の数が純水に比べて幾ら代わったかは判りません。 今仮に酢酸１分子の体積が水１分子の体積と同じだとすると，水分子は酢酸分子の分（1 mmol）だけ減った事になります。つまり，１Ｌ中に 55.5 [mol] － 1 [mmol] ＝ 55.5 － 0.001 [mol] ＝ 55.4999 [mol] ≒ 55.5 [mol] 存在する事になり，純水の場合と同じです。 これは，酢酸１分子の体積が水 100 分子分（そんな事は無いですが）としても同じで，１Ｌ中の水は 55.5 － 0.1 ＝ 55.4 [mol] となり，ほとんど代わりません。 いかがでしょうか。内容は「自信あり」ですが，こういった回答で良いかが判りませんので「自信なし」としておきます。
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完璧な内容の答えだと思います。助かりました。55.4999＝55.5という所まではなんとなくわかってたのですが、そのほかがうまく説明できなかったので助かりました。どうもありがとうございました。