Molar mass is the mass of one mole (6.022 × 10²³ molecules) of a substance, in grams per mole. The Toolenza calculator computes molar mass from a chemical formula by summing the atomic masses of all atoms in the molecule.

Why molar mass is the bridge

Lab chemistry deals in masses (what your balance reads). Stoichiometry — the math of chemical reactions — deals in moles (counts of molecules). Molar mass is the conversion factor:

moles = grams ÷ molar mass

Everything downstream (yield, concentration, reaction balancing) starts with that conversion.

Reference values to recognise

• H — 1.008 g/mol · C — 12.011 · N — 14.007 · O — 15.999 · Na — 22.990 · Cl — 35.453 · K — 39.098 · Ca — 40.078 · Fe — 55.845 · Au — 196.967.
• H₂O — 18.02 g/mol. Every chemistry student memorises this.
• CO₂ — 44.01 g/mol.
• NaCl — 58.44 g/mol.
• C₆H₁₂O₆ (glucose) — 180.16 g/mol.
• C₂H₅OH (ethanol) — 46.07 g/mol.

Worked example — prepping a NaOH solution

Want 250 mL of 0.1 M NaOH? Moles needed = 0.25 L × 0.1 mol/L = 0.025 mol. Mass needed = 0.025 mol × 40.00 g/mol = 1.00 g of NaOH. Weigh it out, dissolve in slightly less than 250 mL of water, transfer to a volumetric flask, top up.

Pitfalls

• Hydrates count the water. Copper sulphate pentahydrate (CuSO₄·5H₂O) has molar mass 249.69 g/mol, not 159.61 (anhydrous). The five waters add up.
• Atomic vs molecular vs formula mass. For ionic compounds (NaCl), it's technically "formula mass" since there are no discrete molecules — but everyone calls it molar mass anyway.
• Isotope-specific applications. Carbon-14 dating needs the C-14 mass (14.003), not the natural-abundance average (12.011). The periodic-table value is a weighted average of natural isotopes.