formelmassa

Formelmassa, also called formula mass or relative formula mass, is the sum of the standard atomic masses of all atoms in a chemical formula. It is typically expressed in atomic mass units (u) and represents the mass contribution of the formula’s constituent atoms.

To calculate the formelmassa, add the atomic masses of each element in the formula, multiplied by the number of times that element appears. For example, the formula mass of water (H2O) is 2 times the atomic mass of hydrogen plus 1 times the atomic mass of oxygen, giving about 18.015 u. For carbon dioxide (CO2), the mass is about 44.009 u (12.011 for carbon plus 2 times 15.999 for oxygen). Sodium chloride (NaCl) has a formula mass of about 58.44 u.

Relation to molar mass: the numerical value of the formula mass closely matches the molar mass in g/mol. This is because 1 atomic mass unit corresponds to 1 g per mole. In practice, the molar mass is the formula mass expressed with units (g/mol). The distinction matters mainly in notation and context; formula mass is in u (dimensionless when stated that way), while molar mass has the explicit unit g/mol.

Notes and caveats: the atomic masses used are often standard atomic weights, which are averages that reflect natural isotopic abundance. For some compounds, especially those with hydrates or when using non-standard isotopic compositions, the value can vary. The hydration state (for example, CuSO4·5H2O) affects the total formula mass if included in the formula being measured.