The Law of Conservation of Mass states that matter cannot be created or destroyed. Thus, the mass of substances produced in a chemical reaction is always equal to the mass of reacting substances. Therefore, you need to have the same number of each type of element on each side of a chemical equation. This is the whole purpose of balancing a chemical equation.
A hint for balancing
Single elements or any diatomic elements, should be left last to balance. This is because whatever coefficient you put in front of the element will not mess up everything that you have already balanced in the equation.
Example:
_ C2H2 + _ O2 -> _ CO2 + _ H2O
Balance all the other elements first and leave oxygen (O2) for last as it is just a single element.
2 C2H2 + _ O2 -> 4 CO2 + 2 H2O
2 C2H2 + 10 O2 -> 4 CO2 + 2 H2O
Polyatomic ions
If the polyatomic ion occurs in the same form on both sides of the equation, treat it as one unit.
Example:
_ Hg(OH)2 + _ H3PO4 -> _ Hg3(PO4)2 + _ H2O
In this case, we treat PO4 as a single unit since it occurs in the same form on both sides of the equation. Although OH is a polyatomic ion, it does not occur in the same form on both sides of the equation, and thus, we do not treat it as a single unit.
_ Hg(OH)2 + 2 H3PO4 -> _ Hg3(PO4)2 + _ H2O
Since we’re treating PO4 as a single unit and there is a subscript of 2 outside the brackets of PO4, the coefficient in front of H3PO4 would be a 2.
3 Hg(OH)2 + 2 H3PO4 -> _ Hg3(PO4)2 + _ H2O
A coefficient of 3 would go in front of Hg(OH)2 because the product is Hg3.
3 Hg(OH)2 + 2 H3PO4 -> Hg3(PO4)2 + 6 H2O
Going through the chemical equation, we can conclude that the coefficient in front of H2O should be a 6 as it balances all the hydrogens and oxygens out in the equation. The coefficient in front of Hg3(PO4)2 would be a 1.
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