9. Mass Calculations from Balanced Equations

Conservation of mass is not just a rule to state — it is a rule you can calculate with.

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Because the total mass of reactants equals the total mass of products, knowing all but one mass lets you find the missing one by adding or subtracting.

Worked example 1. 12 g of carbon reacts completely with 32 g of oxygen in a sealed container. What mass of carbon dioxide forms?
12 + 32 = 44 g.

Worked example 2. 50 g of calcium carbonate decomposes into 28 g of calcium oxide plus carbon dioxide gas. What mass of CO₂ was released?
50 − 28 = 22 g.

⚠️ Common misconception: this only works when you account for every substance, including gases. If a gas escaped, the numbers won't add up — that missing mass is the gas.

💡 Tip: add the reactants, add the products, set them equal. Then solve for the one you don't know.