Conservation of Mass Counting Atoms

• You already know how to read SUBSCRIPTS

• A subscript tells us how many of each atom there are

• Add a subscript of 2 after Oxygen and you make Hydrogen Peroxide.

• The large number in FRONT of the symbol. You have to multiply this number by all of the subscripts.

• 4 Iron Atoms

• 6 Oxygen Atoms

• Multiply the coefficient with each subscript

• 3 x 2 = 6 Nitrogen atoms

• 3 x 4 = 12 Oxygen atoms

​Law of Conservation of Mass

Like energy, mass cannot be created or destroyed.

The amount of mass at the start = the amount of mass at the end

In a chemical reaction, the number of atoms of each element must be balanced.

If you start with 4 Hydrogens, you must end with 4 Hydrogens

​Notice there are equal numbers of Hydrogen atoms and Oxygen atoms on each side.

• Follow the Law of Conservation

• Have the same number and type of atoms on both sides

• They just get rearranged!

​Unbalanced Balanced

Al + O₂ → Al₂O₃ 4Al + 3O₂ → 2 Al₂O₃
Products: Reactants: Products: Reactants:
Al: 1 Al: 2 Al: 4 Al: 4
O: 2 O: 3 O: 6 O: 6

Balancing Equations

You balance equations by adding coefficients in front of the elements and compounds.

You CANNOT change the subscripts!

This equation is unbalanced











H: 2 H: 3
N: 2 N: 1

Start to balance it by adding a 2 in front of NH₃ then recount your atoms









H: 2 H: 3 6
N: 2 N: 1 2

Finish by adding a 3 in front of H₂ then recount your atoms








H: 2 6 H: 3 6
N: 2 N: 1 2