Massaosuus

Massaosuus, or mass fraction, is a way of expressing the proportion of a component by mass in a mixture. For a component i in a mixture, the mass fraction is defined as w_i = m_i / m_total, where m_i is the mass of component i and m_total is the total mass of the mixture. When expressed as a percentage, this is called the mass percent: p_i = w_i × 100%. Mass fractions are dimensionless and, for a closed system, the sum of the fractions of all components equals 1 (or 100%).

Mass fraction is used to describe composition regardless of how the mixture was made. It is related to, but not the same as, mole fraction, which is based on the number of moles rather than mass. Other related measures include mass concentration (mass per unit volume) and, in phase systems, phase mass fractions describing each phase's share of the total mass.

Determination can be performed gravimetrically by weighing components, by measuring mass flows in production, or by analytical techniques that quantify components. In solutions, the mass fraction of the solute is m_solute / (m_solute + m_solvent). For a simple example, a liquid mixture containing 2.0 g of solute and 8.0 g of solvent has a solute mass fraction of 0.20 (20%), and a solvent mass fraction of 0.80 (80%).

Limitations include potential inhomogeneity in mixtures, where mass fractions can vary spatially, and the fact that mass fraction does not convey information about chemical activity or molar properties. Massaosuus provides a straightforward, mass-based description of composition in many scientific and engineering contexts.