Question

how to balance a equation step wise

Answer 1

solve equating the compound having greater mass 1st then go on equating the left hand side and the right hand side
2Mg + O2 --- 2MgO

Answer 2

Step 1: Write down the chemical reaction

Once you know the chemical reaction which you have to balance, write down the unbalanced basic chemical reaction.Usually the chemical reaction will be given.If not, from your understanding of the nomenclature of compounds, figure out the chemical formula of the reactants and products and arrange them on the respective sides of the chemical reaction.

Step 2: Calculate the number of atoms

Once you have the basic unbalanced chemical reaction, count the number of atoms of a particular element on the reactant side as well as the product side.On the basis of the law of conservation of mass, mass is neither created nor destroyed in a chemical reaction.Now we comparing the number of atoms on for a particular element on both sides of the arrow.If they are the same, the equation is already balanced.If they are not the same, the equation is balanced and we need to balance the chemical reaction.

Step 3: Adding coefficients to the molecules to balance the chemical reaction

To balance the chemical reaction, we add coefficients to the molecules rather than to the individual element’s atoms. As adding atoms to the molecules would change the nature of the molecule. Thus we change the number of molecules that are a part of the reaction to balance the chemical reaction.The first step involved would be to balance out a single molecule of reactant & product first.Keep the task of balancing hydrogen and oxygen for last as usually once you are done with the rest, hydrogen and oxygen should be already balanced. Also, oxygen and hydrogen will be fused with other compounds usually, thus trying to balance them first will make the process much more complex.This strategy is called as balancing by inspection.

Step 4: Balance the required amount of Oxygen & Hydrogen 

Once the rest of the reactants are balanced, find the right ratio of oxygen or hydrogen atoms such that the numbers will be equal on both sides of the arrow.

Thus you have learned the theory behind how to balance a chemical reaction. Let us get into a bit more detail and practice with the help of an example.

Example:

Consider the case of rusting of Iron.

Step 1:

We identify the reactants and products. In the case of rusting of iron, we have Iron and oxygen reacting to give iron oxide.

Which can be represented as :

Iron reacting with oxygen to provide iron oxide, the chemical name of rust.

Step 2:

Number of atoms in reactant side:

Fe : 1O : 2

Number of atoms in product side:

Fe : 2O : 3

Step 3:

To balance the number of Fe atoms we compare the reactant and product sides.

We can balance it by giving the iron on the reactant side a coefficient of 2.

But if we consider the case of oxygen as well we can see that we will again have to multiply the reaction by a larger number to make the equation balanced. Thus we give a coefficient of 4 on the reactant side.

Step 4:

Now to balance the number of oxygen atoms. Counting the number of atoms, we can see that the equation can be balanced by having 3 molecules of oxygen each with 2 atoms on the reactant side.