BioDeviceHub

Molecular Weight Calculator

Enter a chemical formula to get its molecular weight and elemental composition, using IUPAC standard atomic weights. Brackets and hydrates are supported.

Formula

MW=∑elementsnatoms×Ar\mathrm{MW} = \sum_{\text{elements}} n_{\text{atoms}} \times A_r
welement (%)=subtotal of the elementMW×100w_{\text{element}}\,(\%) = \dfrac{\text{subtotal of the element}}{\mathrm{MW}} \times 100
Σ\Sigma
sum over every element in the formula

How it works

The formula is read left to right. A number after an element or a closing bracket multiplies what comes before it, so Ca(OH)2 has one calcium, two oxygens and two hydrogens. A dot separates water of hydration: CuSO4·5H2O adds five waters to copper sulfate.

Each element count is multiplied by its IUPAC abridged standard atomic weight and the results are summed. Elements with no stable isotopes, such as technetium, have no standard atomic weight and are reported as such.

Worked example

Find the molecular weight of glucose, C6H12O6.

  1. Carbon: 6 × 12.011 = 72.066.
  2. Hydrogen: 12 × 1.008 = 12.096.
  3. Oxygen: 6 × 15.999 = 95.994.

MW = 72.066 + 12.096 + 95.994 = 180.156 g/mol. Glucose is 40.00% carbon by mass.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • Natural isotopic abundance. Isotopically labelled compounds need the specific isotope masses instead.
  • Atomic weights are IUPAC abridged values to five significant figures, so the result is limited to that precision.

Common mistakes

  • Typing symbols in the wrong case. CO is carbon monoxide; Co is cobalt.
  • Leaving out water of hydration. The hydrate on the shelf weighs more per mole than the anhydrous formula in the protocol.
  • Using a free-acid or free-base weight for a reagent that is supplied as a salt.

Related tools

Molarity CalculatorMass, concentration or volume for a molar solutionOpen calculatorPercent Solution Calculator% w/v, % v/v and % w/w solutionsOpen calculatorBuffer Calculator (Henderson-Hasselbalch)Acid and base amounts for a buffer at a target pHOpen calculator