Conservation of Mass

Objective: Explain the concepts and assumptions used in conservation of mass.

For ordinary chemical reactions, a closed matter boundary retains the same total mass to laboratory precision. Gases count as matter, and an open vessel may lose or gain mass through gas transfer. Define the system before interpreting a measurement. Product mass includes all retained substances, including solvent or unreacted material when those were included initially. The simplified exercises specify complete conversion and only the stated components; actual experiments require fuller accounting.

Objective: Apply conservation of mass to calculations, evidence, and specified situations.

For the application problems, define the system and list the quantities supplied before choosing a relation. Use initial total mass = final total mass for a closed matter account. The relation must match the stated conditions. Convert to compatible units before substitution, preserve intermediate precision, and report the requested quantity with its unit or sign. When comparing alternatives, check the defining condition rather than a superficial word match. The worked examples below show the setup and calculation separately.