Because atoms are rearranged rather than created or destroyed in ordinary reactions, total mass is conserved. In a closed system, no matter crosses the boundary, so the mass before and after a reaction is equal. Add all reactant masses and all product masses, including any substances left over. To find one missing mass, subtract the known masses from the conserved total.
An open container may appear to lose mass when a gas escapes. Burning magnesium may gain mass because oxygen enters from the surrounding air. Neither observation breaks conservation: the system boundary excluded matter that moved in or out. A balanced equation tracks atoms; a complete mass account tracks the same conservation at a measurable scale. Do not assume equal masses of each individual reactant and product.
Example 1
In a sealed vessel, 12g of carbon reacts completely with 32g of oxygen. What mass of carbon dioxide forms?
The only product has mass 12+32=44g. The sealed system keeps all matter inside.
Example 2
A 5g metal sample forms 8g of oxide. What mass of oxygen joined the metal?
Oxygen mass = oxide mass − original metal mass =8−5=3g. The oxygen came from the air.
Watch out: Escaping gas carries matter outside the measured boundary. Include the gas and mass is conserved.