Fill in the coefficients that will balance the following reaction
a0h2+a1o2=a2h2o

Balancing is making the number of atoms of each element same on both the sides  (reactant and product side).

To find the number of atoms of each element we multiply coefficient × subscript  

For example   contains  

5 × 1 = 5 , Mg atoms and

5 × 2 = 10, Cl atoms  

If there is a bracket in the chemical formula  

For example we multiply coefficient × subscript × number outside the bracket.......... to find the number of atoms  (Please note: 3 is the coefficient, and if there is no number given then 1 will be the coefficient )

So 3 × 3 = 9, Mg atoms  

3 × 1 × 2 = 6, P atoms  

3 × 4 × 2 = 24, O atoms are present.

So

Let us balance the given equation  

(Unbalanced)

Reactant Side       # of atoms of each element      Product Side

2                                            H                                           2

2                                           O                                           1

Since O atoms are not balanced on the right side we multiply by 2 so the equation changes to

Reactant Side       # of atoms of each element      Product Side

2                                            H                                           4

2                                           O                                           2

Now to balance H atoms on the reactant side we multiply by 2 so we will have

 (Balanced)

Reactant Side       # of atoms of each element      Product Side

4                                            H                                           4

2                                           O                                           2

Balanced!!!

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