Whenever you use a "shortcut" in chemistry, you increase the likelihood for a mistake. However, there is one shortcut that can save lots of time when balancing chemical equations with polyatomic ions.

If a chemical equation has the SAME polyatomic ions on BOTH sides of the arrow, balance the equation by balancing the ions.

Obviously, these equations will be Double Displacement equations.

Notice that the equation below has the nitrate and sulfate polyatomic ions on both sides.

Al(NO₃)₃ + H₂SO₄ → HNO₃ + Al₂(SO₄)₃

There are three nitrates on the left, and only one nitrate on the right.

There are three sulfates on the right, and only one sulfate on the left.

The sulfates can be balanced by placing a coefficient of three in front of the sulfuric acid on the left.

Al(NO₃)₃ + 3 H₂SO₄ → HNO₃ + Al₂(SO₄)₃

The nitrates can be balanced by placing a coefficient of three in front of the nitric acid on the right.

Al(NO₃)₃ + 3 H₂SO₄ → 3 HNO₃ + Al₂(SO₄)₃

We have balanced the polyatomic ions, but what about aluminum and hydrogen? They ARE NOT balanced.

Balance the aluminum by placing a 2 in front of the aluminum nitrate on the left. This ALSO changes the number of nitrates on the left from 3 to 6.

2 Al(NO₃)₃ + 3 H₂SO₄ → 3 HNO₃ + Al₂(SO₄)₃

The nitrates can be fixed by changing the 3 in front of the nitric acid to a 6. This also makes 6 hydrogen atoms on both sides.

2 Al(NO₃)₃ + 3 H₂SO₄ → 6 HNO₃ + Al₂(SO₄)₃  

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Here is an equation with THREE polyatomic ions on both sides.

(NH₄)₃PO₄ + Ba(OH)₂ → NH₄OH + Ba₃(PO₄)₂

Balance the NON polyatomic ion first.

(NH₄)₃PO₄ + 3 Ba(OH)₂ → NH₄OH + Ba₃(PO₄)₂

Now the hydroxides.

(NH₄)₃PO₄ + 3 Ba(OH)₂ → 6 NH₄OH + Ba₃(PO₄)₂

Now the ammoniums.

2 (NH₄)₃PO₄ + 3 Ba(OH)₂ → 6 NH₄OH + Ba₃(PO₄)₂

The phosphates are already balanced.