Question

How to calculate molecular weight

Answer 1

"Molecular weight" (more correctly called molar mass or relative molecular mass) is the sum of the atomic masses of the atoms in a molecule. It is usually reported in grams per mole (g·mol⁻¹) and numerically equals the average molecular mass in daltons (Da).

Step-by-step method

1. Write the chemical formula of the compound.
2. Use a periodic table to find the atomic mass of each element (use the average atomic mass listed; units: atomic mass units u or g·mol⁻¹).
3. Multiply each element’s atomic mass by the number of atoms of that element in the formula.
4. Add those values for all elements in the molecule. The result is the molar mass (g·mol⁻¹).

Examples

- Water, H2O:
- H = 1.008 g·mol⁻¹, O = 16.00 g·mol⁻¹
- M = 2×1.008 + 1×16.00 = 18.016 g·mol⁻¹

- Glucose, C6H12O6:
- C = 12.011, H = 1.008, O = 16.00
- M = 6×12.011 + 12×1.008 + 6×16.00 = 180.156 g·mol⁻¹

- Calcium phosphate, Ca3(PO4)2:
- Ca = 40.078, P = 30.974, O = 16.00
- For formula: 3 Ca, 2 P, 8 O (because PO4 twice)
- M = 3×40.078 + 2×30.974 + 8×16.00 = 310.174 g·mol⁻¹

Notes and tips

- For ionic compounds you may see the term "formula weight" instead of molecular weight; the calculation is the same (sum atomic masses for the empirical formula).
- Hydrates: include water molecules shown in the formula. Example CuSO4·5H2O — add 5×(molar mass of H2O).
- Use the average atomic masses from the periodic table for typical molar mass calculations. If you need exact masses for mass spectrometry, use isotopic masses and calculate based on isotopic composition.
- Units: molar mass in g·mol⁻¹; atomic masses are in u (Da), and 1 u = 1 g·mol⁻¹ numerically.
- Significant figures: round your final result consistent with the precision of atomic masses used.
- Tools: online calculators, chemical databases (PubChem), and software (ChemDraw) can compute molar masses automatically.

If you tell me a specific formula, I can calculate its molecular weight step by step.