Computing molar mass (molar weight)

• Any chemical element. Capitalize the first letter in chemical symbol and use lower case for the remaining letters: Ca, Fe, Mg, Mn, S, O, H, C, N, Na, K, Cl, Al.
• Functional groups: D, T, Ph, Me, Et, Bu, AcAc, For, Tos, Bz, TMS, tBu, Bzl, Bn, Dmg
• parenthesis () or brackets [].
• Common compound names.

Computing molecular weight (molecular mass)

Definitions

• Molecular mass (molecular weight) is the mass of one molecule of a substance and is expressed in the unified atomic mass units (u). (1 u is equal to 1/12 the mass of one atom of carbon-12)
• Molar mass (molar weight) is the mass of one mole of a substance and is expressed in g/mol.
• Mole is a standard scientific unit for measuring large quantities of very small entities such as atoms and molecules. One mole contains exactly 6.022 ×10²³ particles (Avogadro's number)

Steps to calculate molar mass

1. Identify the compound: write down the chemical formula of the compound. For example, water is H₂O, meaning it contains two hydrogen atoms and one oxygen atom.
2. Find atomic masses: look up the atomic masses of each element present in the compound. The atomic mass is usually found on the periodic table and is given in atomic mass units (amu).
3. Calculate molar mass of each element: multiply the atomic mass of each element by the number of atoms of that element in the compound.
4. Add them together: add the results from step 3 to get the total molar mass of the compound.

Example: calculating molar mass

Let's calculate the molar mass of carbon dioxide (CO₂):

• Carbon (C) has an atomic mass of about 12.01 amu.
• Oxygen (O) has an atomic mass of about 16.00 amu.
• CO₂ has one carbon atom and two oxygen atoms.
• The molar mass of carbon dioxide is 12.01 + (2 × 16.00) = 44.01 g/mol.

Lesson on computing molar mass

Weights of atoms and isotopes are from NIST article.

Related: Molecular weights of amino acids