Conserve Atoms When Balancing Equations

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A coefficient multiplies every atom in a formula. A subscript belongs to the substance and must not be changed to balance an equation. Balance the count of each element, not just the number of molecules.

Worked example

2H2 + O2 -> 2H2O has four H atoms and two O atoms on each side. The leading 2 multiplies the whole water formula. This is a paper exercise, not an experiment.

1. How many oxygen atoms are represented by 3CO2? Answer

2. How many hydrogen atoms are represented by 4H2O? Answer

3. Balance __Mg + O2 -> 2MgO. What coefficient belongs before Mg? Answer

4. Why is changing H2O to H2O2 not an acceptable balancing step? It changes the identity of the substance Subscripts never affect atom counts It changes only the printed size

5. Which equation is balanced? Na + Cl2 -> NaCl 2Na + Cl2 -> 2NaCl 2Na + 2Cl2 -> 2NaCl