The key in
the balancing of chemical reaction is to make sure that you got the exact
chemical formula of the substance that is reacting and the yielded product.
After getting the formula, identify first the hardest element/atom to be
balanced such as Na, Zn, Cl, C, O, etc. Then proceed to the easier ones such as
O, H. Make sure to note the atoms present in the molecule of the substance. Among
the three equations, PCl2 + 4H2O --->H3PO4 + 5HCL is not balanced. The
hardest element here is the Cl. Observe that there are 5Cls in the product side
so you must add 5 to the PCl2. However, there are only 2Cl atoms and it needs
3 to balance the equation. Also, there is no P in the product side. This equation
is invalid.
