The mathematical relationship you should use to convert moles of a substance into grams is the molar mass.

Here's the breakdown:

* Molar mass is the mass of one mole of a substance. It is expressed in grams per mole (g/mol).

* To convert moles to grams, you simply multiply the number of moles by the molar mass of the substance.

Formula:

* Mass (g) = Moles (mol) x Molar Mass (g/mol)

Example:

Let's say you have 2 moles of water (H₂O). The molar mass of water is 18.015 g/mol.

* Mass (g) = 2 mol x 18.015 g/mol = 36.03 g

Therefore, 2 moles of water weigh 36.03 grams.