How do you balance the chemical equations for double replacement reactions?
1 Answer
You balance double replacement equations by pairing each cation with the anion from the other compound in the correct ratios.
Explanation:
Example
Write a balanced equation for the reaction between Fe(NO₃)₂ and Na₃PO₄.
Solution

Identify the cations and anions in each compound:
Fe(NO₃)₂ has Fe²⁺ and NO₃⁻
Na₃PO₄ has Na⁺ and PO₄³⁻ 
Pair up each cation with the anion from the OTHER compound:
Fe²⁺ pairs with PO₄³⁻
Na⁺ pairs with NO₃⁻ 
Write two new (CORRECT!!) formulas using the pairs from
Step 2.
Fe₃(PO₄)₂, since Fe is +2 and PO₄ is 3
NaNO₃ since Na is +1 and NO₃ is 1 
Write the unbalanced equation
Fe(NO₃)₂ + Na₃PO₄ → Fe₃(PO₄)₂ + NaNO₃ 
Balance the equation.
The Fe₃(PO₄)₂ has 3 Fe and 2 PO₄ on the right, so you need 3 Fe and 2 PO₄ on the left:
3Fe(NO₃)₂ + 2Na₃PO₄ → Fe₃(PO₄)₂ + NaNO₃
Now we have 6 NO₃ on the left, so we need 6 NO₃ on the right.
The balanced equation is
3Fe(NO₃)₂ + 2Na₃PO₄ → Fe₃(PO₄)₂ +6NaNO₃
Here is a video to help with balancing double replacement reactions.