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. Write the unbalanced chemical equation. 
Left would be the reactant side, while right would be the product side. 
C3H8 + O2 ---> H2O + CO2 

2. Start by balancing elements which is NOT Hydrogen or Oxygen. Hydrogen should be second to the last and Oxygen last. 
As we can see, Carbon is not balanced. There are 3 carbon atoms while in the product side, only one. So to balance it, we need to write 3 as a stoichiometric coefficient. It is the number in the front of a compound. 
This will now be: 
C3H8 + O2 ---> H2O + 3CO2 

3. Next, we're now only left with H and O, we must first balance Hydrogen. In the reactant side, there are 8 H atoms, while, there are only 2 H atoms at the right. So in order to make 2, eight, we need to multiply it by 4. So you have to put a coefficient in front of H in the product side. 
C3H8 + O2 ---> 4H2O + 3CO2 

4. Now, we finally balance O. In the left, there are 2 atoms while at the right, Oxygen has 4 atoms in H2O plus 6 atoms in CO2 (3*2) so at the product side, we have 10 atoms at the product side. 
To balance it, we'll need to multiply 2 by 5 in the reactant side. 
C3H8 + 5O2 ---> 4H2O + 3CO2 
hope it helps a bit!!!
edited the ans
herez the url
She mean in 10th book.
A chemical equation is a written symbolic representation of a chemical reaction. The reactant chemical(s) are given on the left-hand side and the product chemical(s) on the right-hand side. The law of conservation of mass states that no atoms can be created or destroyed in a chemical reaction, so the number of atoms that are present in the reactants has to balance the number of atoms that are present in the products. Follow this guide to learn how to balance chemical equations differently.
Write the unbalanced equation.
Balance the equation.
Indicate the states of matter of the reactants and products.
 Example Problem:
1.Write the unbalanced equation.SnO2 + H2 → Sn + H2O

Balance the equation.Look at the equation and see which elements are not balanced. In this case, there are two oxygen atoms on the lefthand side of the equation and only one on the righthand side. Correct this by putting a coefficient of 2 in front of water:SnO2 + H2 → Sn + 2 H2OThis puts the hydrogen atoms out of balance. Now there are two hydrogen atoms on the left and four hydrogen atoms on the right. To get four hydrogen atoms on the right, add a coefficient of 2 for the hydrogen gas. Remember, coefficients are multipliers, so if we write 2 H2O it denotes 2x2=4 hydrogen atoms and 2x1=2 oxygen atoms.SnO2 + 2 H2 → Sn + 2 H2O

Indicate the physical states of the reactants and products.To do this, you need to be familiar with the properties of various compounds or you need to be told what the phases are for the chemicals in the reaction. Oxides are solids, hydrogen forms a diatomic gas, tin is a solid, and the term 'water vapor' indicates that water is in the gas phase:SnO2(s) + 2 H2(g) → Sn(s) + 2 H2O(g)

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