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March 29, 2017

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In the situation that a solute dissolves in a solvent, the solute molecules often dissociate and associate. Studying for exam, not sure how to explain HOW dissociation and association affects the molar mass of the solute in this system.

(What is dissociation and association?)

  • Chemistry - ,

    Dissociation is almost the same as ionization; for example, NaCl in H2O dissociates into Na^+ and Cl^-.
    Acetic acid ionizes into
    CH3COOH + H2O ==> H3O^+ + CH3COO^-

    Association is the reverse of dissociation; i.e., positive and negative ions come together. An example is acetic acid associates, due to hydrogen bonding. As a result, the molar mass of acetic acid may be somewhere between 60 (when not associated) to 2x or 3x that number (120 or 180) especially in concentrated solutions. In some experiments I have done, the molar mass of acetic acid was 90 or the equivalent of about 2 molecules out of every 3 becoming associated. Hope this helps.

  • Chemistry - ,

    Thanks you Dr.Bob222.
    So the overall idea is that dissociation decreases molar mass and association does the opposite when a solute dissolves in a solvent. (How do you identify if either reaction occurs? Just wondering.)

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