The reason you and others get confused by concentrations are that concentrations by definition are unit-less. Concentrations are often expressed as parts per some number of (air, water, blood) etc. Parts per million or parts per billion etc. When concentrations are expressed as units of mass such as grams or micrograms or nanograms per liter etc, the molecular weight of the substance must be used to convert the concentration. This is why the same concentration of different chemicals when expressed as a mass may be different.
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