Question

58.5amu are termed as formula mass and not molecular mass of Nacl.Why?

Answer 1

Because NaCl is an IONIC compound, and not a molecular, which means it doesn't form molecules, but formula units. 
58.8 amu is the mass of the smallest number of particles that can form NaCl (and that is one sodium and one chlorine atom). 

Answer 2

Answer:

Because NaCl is an ionic substance and molecules of ionic substances are termed as formula units but as molecular mass. That's why 58.5 amu is the formula mass of NaCl and not molecular mass.

Explanation:

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