In this page, we will look at some examples of applying
Rule 5: Balancing chemical equations containing polyatomic ions
Case 1: If the polyatomic ion remains the same before and after the reaction, then treat it as “a single element” for ease of calculation. Try to start with the most complicated-looking group.
Example:
Balance the equation
Ca + HNO3 → H2 + Ca(NO3)2
Solution:
The nitrate ion NO3 is unchanged before and after the reaction.
Start with Ca(NO3)2 since it looks most complicated.
There is one NO3 on the left and two on the left.
Using the one’s and two’s technique, the equation is balanced by placing the coefficient of 2 for HNO3
Ca + 2HNO3 → H2 + Ca(NO3)2
We test the other atoms and we find that we already have a balanced equation.
Check to make sure that all coefficients are in the lowest-possible ratio
The following videos show some examples of balancing chemical equations containing polyatomic ions.
Case 2: If the polyatomic ion is changed after the reaction then it would be necessary to consider each atom individually.
Example:
Balance the chemical equation
Ba(OH)2 + H3PO4 → BaHPO4 + H2O
Solution:
Use the CHO technique. Since we don't have carbon, we could try to balance the hydrogen first.
We have five hydrogen atoms on the left and three hydrogen atoms on the right.
To balance the hydrogen, we can place the coefficient of 2 at H2O
Ba(OH)2 + H3PO4 → BaHPO4 + 2H2O
We test the other atoms and we find that we already have a balanced equation.
Check to make sure that all coefficients are in the lowest-possible ratio.
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