1. Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq) Objectives
To be able to write a balanced equation for a chemical reaction
Pb(NO3)2 (aq) + 2NaI (aq)
PbI2 (s) + 2NaNO3 (aq)

2. A chemical reaction is balanced by using a systematic approach:
Write the formulas of the reactants and products to give the unbalanced chemical equation.
Balance by trial and error starting with the most complicated molecule(s). Leave pure elements to the end.
At the end check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides).

3. Example: 
Hydrogen gas and oxygen gas combine to form liquid water.
Write the formulas of the reactants and products to give the unbalanced chemical equation. H2(g) + O2(g)  H2O(l)

4. Hydrogen gas and oxygen gas combine to form liquid water.

5. Example: 
Hydrogen gas and oxygen gas combine to form liquid water.
At the end check to be sure the equation is balanced (same numbers of all types of atoms on the reactant and product sides). 2H2(g) + O2(g)  2H2O(l)