Final answer:
The relation between atomic mass unit (amu) and grams (g) is through molar mass, where the mass of one molecule in amu is numerically equal to the mass of one mole of the substance in grams.
Step-by-step explanation:
The atomic mass unit (amu) and the grams unit (g) are related to one another through the concept called the molar mass. The amu is a small unit of mass used to measure the mass of atoms and subatomic particles. For practicality and ease of comparison, the atomic mass of an element is often expressed relative to a standard, which is the mass of the carbon-12 isotope. By definition, one-twelfth of the mass of a carbon-12 atom is equal to one amu.
An interesting relationship exists between the mass of one molecule in amu and the mass of one mole of that substance in grams (g). For any substance, the numerical value of its molecular mass in amu is the same as the numerical value of its molar mass in grams per mole. Therefore, if a single molecule of a substance has a mass of 18 amu, like water (H₂O), then the molar mass of water is 18 g/mol.