There's no direct conversion between grams and millimoles. To convert between them, you need the molecular weight (also called molar mass) of the substance you're working with.

Here's the breakdown:

* Millimole (mmol): A unit of amount of substance, equal to one-thousandth of a mole.

* Mole (mol): The standard unit of amount of substance in the International System of Units (SI). It represents a specific number of particles (6.022 x 10^23, Avogadro's number).

* Molecular Weight (g/mol): The mass of one mole of a substance, expressed in grams per mole.

Conversion Formula:

```

grams = millimoles * molecular weight / 1000

```

Example:

Let's say you have 5 millimoles of sodium chloride (NaCl). The molecular weight of NaCl is 58.44 g/mol.

* grams = 5 mmol * 58.44 g/mol / 1000

* grams = 0.2922 g

Therefore, 5 millimoles of sodium chloride is equal to 0.2922 grams.