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?)
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.
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.)
Dissociation is a process in which a compound breaks down into its component ions when dissolved in a solvent. The compound's molecules separate into individual ions. For example, if we dissolve table salt (NaCl) in water, the NaCl molecules dissociate into individual sodium (Na+) and chloride (Cl-) ions.
Association, on the other hand, is the opposite process. It occurs when ions or molecules in a solvent come together to form a compound. For example, when water vapor condenses to form liquid water, the individual water molecules associate with each other to form a larger structure.
Now, let's discuss how dissociation and association affect the molar mass of a solute.
When a solute dissociates, the molar mass of the solute appears to decrease. This is because the molar mass of the solute is based on the sum of its constituent atoms or ions. When the compound dissociates, the individual ions are now counted separately instead of being combined as part of a single molecule. For example, when NaCl dissociates in water, the molar mass of the solute NaCl is the sum of the molar masses of individual sodium and chloride ions.
On the other hand, when a solute associates, the molar mass of the solute appears to increase. This happens because the molar mass of the solute is based on the sum of its constituents, and when the individual ions or molecules associate to form a compound, the molar mass is calculated based on the new structure. For example, when water vapor associates to form liquid water, the molar mass of the solute water increases because the individual water molecules are now combined to form a larger structure.
In summary, dissociation of a solute decreases the apparent molar mass, whereas association of a solute increases the apparent molar mass. These processes affect the molar mass calculation by considering the individual ions or molecules produced or formed when the solute dissolves in a solvent.