To balance a chemical equation, you first must count the number of each of the atoms on both the left hand side - reactant side and the right hand side - product side of the chemical reaction.
After counting the atoms, make a tally of the atoms on both sides, for instance if it is hydrogen - h, record the number of hydrogen atoms on the reactant and product side. Then, place coefficients at appropriate places for the elements and or compounds, to basically have all of the different kinds of atoms from their respective elements equal the same on both the left and right side of the reaction.
SnO2 + H2 > Sn + H2O 1st count the metals - Sn atoms on both sides - Sn left side - 1 Sn right side - 1
2nd count the oxygen atoms on both sides O atom left side - 2 O atom right side - 1.
Here, the atoms are not the same, so place a coefficient on the side with the least number of oxygen atoms, right side, put 2 to balance.
Now O atoms are same on both sides, 2 atoms but, there are 4 hydrogen atoms now,
Since 2H2O = 2H2 + 2O = H = 4 and O = 2.
Now place the coefficient of 2 on the side with Hydrogen atoms by itself, left side, now this also gives us 4 hydrogens.
