Some liquids have enough attractions between molecules to form dimers. (dimers are molecules formed from the combination of identical molecules, a+ a → a2.) what effect would this have on the experimental molar mass?

The experimental molar mass doubles.

Explanation:

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In this case, dimers are understood as the gathering of two monomeric units of a polymerizable molecule, thus, if a molecule A monomer has a specific molar mass, say, 100g/mol determined experimentally, if it forms a dimer, its molar mass get doubled since the resulting dimer will be A₂, therefore the assumed experimental molar mass will be 200 g/mol since two A molecules were gathered.

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