Some common ions exist as groups of atoms.
As the example below shows, for compounds containing more than one of these ions, the formula for the ion is enclosed in brackets, and the subscript is placed outside the brackets. When counting the number of atoms for these be sure to multiply the number outside the bracket by any subscripts inside the brackets.
For some reactions the same ion is present in both reactants and products. If this is the case, it is convenient to consider these groups of atoms as a block for balancing purposes as shown in the example below.
Common examples:
Sulfate (SO42–), nitrate (NO3–),
phosphate (PO43–), hydroxide (OH–)
Sulfate (SO42–), nitrate (NO3–),
phosphate (PO43–), hydroxide (OH–)
As the example below shows, for compounds containing more than one of these ions, the formula for the ion is enclosed in brackets, and the subscript is placed outside the brackets. When counting the number of atoms for these be sure to multiply the number outside the bracket by any subscripts inside the brackets.
Zn(NO3)2 has 1 Zn, 2N and 6 O.
For some reactions the same ion is present in both reactants and products. If this is the case, it is convenient to consider these groups of atoms as a block for balancing purposes as shown in the example below.
Zn(OH)2(aq) + HNO3(aq) 
Zn(NO3)2(aq) + H2O(l)
Zn is balanced.
There is one NO3 group in reactants and two in products.
Zn(OH)2(aq) +2HNO3(aq) Zn(NO3)2(aq) + H2O(l)
4 H in reactants and 2 H in products
Zn(OH)2(aq) + 2HNO3(aq) Zn(NO3)2(aq) + 2H2O(l)
Check for oxygen balance (8 O on both sides).
Zn(NO3)2(aq) + H2O(l)Zn is balanced.
There is one NO3 group in reactants and two in products.
Zn(OH)2(aq) +2HNO3(aq)
Zn(NO3)2(aq) + H2O(l)4 H in reactants and 2 H in products
Zn(OH)2(aq) + 2HNO3(aq)
Zn(NO3)2(aq) + 2H2O(l)Check for oxygen balance (8 O on both sides).