Moles

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A mole is chemistry’s way of counting particles that are far too small and too many to count one by one: one mole is 6.02214076 × 10²³ of them, whether atoms, molecules or ions. A chemistry answer is often in moles (mol), and a blood test reports some results in thousandths of one, millimoles (mmol). The two convert to each other and add together.

1 mol in mmol // 1,000.00 mmol
250 mmol in mol // 0.25 mol
2 mol + 500 mmol // 2.50 mol

The boundary

The mole is a conversion unit only. Turning moles into grams needs the mass of one mole of the particular substance (its molar mass), which the engine does not know, so a mole does not convert to a mass, and multiplying it by other quantities is refused rather than invented into a compound unit.

1 mol in g // ERROR: an amount of substance cannot be converted to a mass
2 mol * 3 kg // ERROR: amount of substance and mass cannot be multiplied

It is left out of the dimensional arithmetic that names a newton or a watt (see named derived units), which tracks mass, length, time and current; the amount of substance is a fifth base quantity outside those, and no named unit the engine produces is made from it. The word mole is not read as the unit, since it is also an animal, a spy and a mark on the skin.