By Sean Lancaster

Updated Aug 30, 2022

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In laboratory settings, chemical amounts are typically expressed in grams. However, reaction stoichiometry requires moles—a unit representing a fixed number of molecules (Avogadro’s constant, 6.02 × 10²³). To determine the mass corresponding to a specific number of moles, you must first know the compound’s formula weight.

Step 1: Find the Formula Weight

Calculate the formula weight by summing the atomic weights of all atoms in the molecular formula. The atomic weights are listed on the NIST periodic table or any standard periodic table.

Example – Iron(III) oxide (rust): Fe₂O₃

• Atomic weight of Fe: 55.845 g mol⁻¹
• Atomic weight of O: 16.000 g mol⁻¹

Formula weight = 2 × 55.845 + 3 × 16.000 = 111.69 + 48.00 = 159.69 g mol⁻¹ (the mass of one mole of Fe₂O₃).

Step 2: Convert to Millimoles

One millimole equals one thousandth of a mole. Divide the formula weight by 1,000.

1 mol Fe₂O₃ = 159.69 g → 1 mmol Fe₂O₃ = 159.69 g / 1,000 = 0.1597 g = 159.69 mg.

Step 3: Calculate the Required Mass

Multiply the desired millimoles by the mass per millimole.

Example: 15 mmol Fe₂O₃ needed → 15 × 159.69 mg = 2,395.35 mg of Fe₂O₃.

What You’ll Need

• Access to a reliable periodic table (e.g., NIST)
• Scientific calculator or spreadsheet