Name the quantity before comparing the number
IUPAC defines mass concentration using a constituent’s mass and the mixture’s volume. Amount concentration instead uses amount of substance and volume, commonly expressed in moles per litre. These definitions explain why grams per litre and moles per litre are not interchangeable labels.
An original arithmetic example
Suppose a hypothetical substance has a molar mass of 200 grams per mole and a reported mass concentration of 2 grams per litre. Dividing 2 grams per litre by 200 grams per mole gives 0.01 moles per litre. The gram units cancel. This is a dimensional-analysis example, not a preparation recipe or a calculation for any catalog compound.
Equal mass does not guarantee equal amount
Now imagine a second hypothetical substance with twice that molar mass. At the same mass concentration, its amount concentration would be half as large. The volume denominator stayed the same; the relationship between mass and amount changed. A comparison that ignores the identity of the substance would miss this difference.
Treat a blend as a collection of constituents
Before attempting a comparison, write down exactly which constituent the number describes. A blend’s total mass does not tell you each ingredient’s share. If those shares are missing, leave the individual values unknown. More decimal places cannot replace missing composition information, and a tidy calculation cannot certify the contents of a container.
The scope of the references
The complete indexed official entries were reviewed in IUPAC’s 2025 online edition; direct automated requests returned an access error. They define quantities rather than test a product. The worked arithmetic and reading checklist are S7 explanations. No personal dosing or administration guidance is provided.