By Chris Deziel Updated Mar 24, 2022
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To determine a compound’s molecular mass, you must know its molecular formula and the atomic mass of each constituent element. Atomic masses, expressed in atomic mass units (amu), are listed under each element’s symbol in the periodic table. By definition, the atomic mass of an element equals the mass of one mole of that element in grams. A mole contains Avogadro’s number (6.02 × 1023) of atoms or molecules.
Add the atomic masses of all atoms in a molecule’s formula to obtain the mass of one mole in grams.
Atoms bond according to the distribution of valence electrons. Ionic compounds, such as sodium chloride (NaCl), consist of one atom of each of two different elements. Covalent gases like hydrogen (H₂) and oxygen (O₂) contain two identical atoms. Complex molecules, particularly organic compounds, can contain dozens of atoms; for example, glucose (C₆H₁₂O₆) contains 24 atoms.
Regardless of size, the calculation method is identical: look up each element’s atomic mass, multiply by its count in the formula, then sum all contributions. The result is the molar mass in grams.
Elements are arranged in the periodic table by increasing atomic number, which equals the number of protons in the nucleus. Hydrogen, with one proton, is first; oxygen, with eight protons, is eighth. Atomic number differs from atomic mass because neutrons also contribute to mass. Electrons are negligible in mass. The atomic mass—sum of protons and neutrons—is listed beneath each element’s symbol.
Round to the Nearest Integer: Atomic mass values often include decimals due to natural isotopic variation. For most practical purposes, round to the nearest whole number. For instance, oxygen’s listed mass is 15.999; rounding gives 16, meaning one mole of oxygen weighs 16 g.
What is the molecular mass of glucose in grams?
Glucose’s formula is C₆H₁₂O₆. From the periodic table, carbon (C) = 12 amu, hydrogen (H) = 1 amu, oxygen (O) = 16 amu. The molar mass equals (6 × 12) + (12 × 1) + (6 × 16) = 180 g/mol. Thus one mole of glucose has a mass of 180 g. To find the mass of multiple moles, multiply 180 g by the number of moles.
