Molar mass is one of the most fundamental concepts in chemistry. It links the microscopic world of atoms and molecules to the macroscopic quantities we can weigh on a balance. Once you understand how to calculate molar mass, stoichiometry — the art of working out quantities in chemical reactions — becomes straightforward.
What Is Molar Mass?
The molar mass of a substance is the mass of one mole of that substance, expressed in grams per mole (g/mol). One mole contains exactly 6.022 × 10²³ particles (Avogadro's number). The molar mass of an element is numerically equal to its relative atomic mass (the number found on the periodic table).
For example: carbon (C) has an atomic mass of 12.011 u, so its molar mass is 12.011 g/mol. One mole of carbon atoms weighs 12.011 g.
Step 1: Read the Chemical Formula
To calculate the molar mass of a compound, start with its chemical formula. The formula tells you which elements are present and how many atoms of each are in one formula unit.
- H₂O = 2 hydrogen atoms + 1 oxygen atom
- NaCl = 1 sodium atom + 1 chlorine atom
- C₆H₁₂O₆ = 6 carbon + 12 hydrogen + 6 oxygen atoms
- Ca(OH)₂ = 1 calcium + 2 oxygen + 2 hydrogen atoms
Step 2: Look Up Atomic Masses
Find the relative atomic mass of each element from the periodic table. The most commonly needed values:
- H = 1.008 g/mol
- C = 12.011 g/mol
- N = 14.007 g/mol
- O = 15.999 g/mol
- Na = 22.990 g/mol
- Mg = 24.305 g/mol
- S = 32.06 g/mol
- Cl = 35.45 g/mol
- Ca = 40.078 g/mol
- Fe = 55.845 g/mol
- Cu = 63.546 g/mol
Step 3: Multiply and Add
Multiply each element's atomic mass by the number of times it appears in the formula, then sum all contributions.
Example 1: Water (H₂O)
M(H₂O) = 2 × 1.008 + 1 × 15.999 = 2.016 + 15.999 = 18.015 g/mol
Example 2: Table Salt (NaCl)
M(NaCl) = 22.990 + 35.45 = 58.44 g/mol
Example 3: Glucose (C₆H₁₂O₆)
M = 6 × 12.011 + 12 × 1.008 + 6 × 15.999
M = 72.066 + 12.096 + 95.994 = 180.156 g/mol
Example 4: Calcium Hydroxide Ca(OH)₂
Note the bracket: the subscript 2 applies to the entire (OH) group.
M = 40.078 + 2 × (15.999 + 1.008) = 40.078 + 2 × 17.007 = 40.078 + 34.014 = 74.092 g/mol
Example 5: Sulfuric Acid (H₂SO₄)
M = 2 × 1.008 + 32.06 + 4 × 15.999 = 2.016 + 32.06 + 63.996 = 98.072 g/mol
Using Molar Mass in Calculations
Converting Grams to Moles
n = m / M
How many moles are in 49 g of H₂SO₄?
n = 49 / 98.072 = 0.5 mol
Converting Moles to Grams
m = n × M
What mass of glucose contains 0.25 mol?
m = 0.25 × 180.156 = 45.04 g
Hydrates — A Common Complication
Hydrated salts contain water of crystallisation. Copper(II) sulfate pentahydrate is written CuSO₄·5H₂O. Its molar mass includes the 5 water molecules:
M = 63.546 + 32.06 + 4 × 15.999 + 5 × 18.015 = 159.606 + 90.075 = 249.681 g/mol
Common Mistakes
- Forgetting to apply subscripts outside brackets (e.g., Ca(OH)₂ has 2 oxygens and 2 hydrogens, not 1 of each)
- Using integer atomic masses instead of the accurate decimal values from the periodic table
- Confusing molar mass (g/mol) with relative molecular mass (dimensionless but numerically identical)
- Forgetting water molecules in hydrated compounds
Calculate molar mass instantly for any element or compound using the chemistry calculators at Liczbnik.pl — just enter the formula and get the result with a full breakdown by element.