The mass of one mole of a substance is equal to:

The mass of one mole of a substance is equal to the molecular weight of that substance, expressed in grams. This is known as molar mass. Here are the steps to calculate it:

1. Identify the substance: The substance could be an element or a compound.

2. Determine the molecular or formula weight: If the substance is an element, the atomic weight is the molecular weight. If the substance is a compound, the molecular weight is the sum of the atomic weights of all the atoms in a molecule of the compound. You can find the atomic weights on the periodic table.

3. Convert molecular weight to grams: The molecular weight is usually given in atomic mass units (amu). To convert it to grams, you use Avogadro's number (6.022 x 10^23), which is the number of particles in one mole. The molecular weight in amu is numerically equal to the molar mass in grams.

So, the mass of one mole of a substance is equal to the molecular weight of the substance in grams.