Balancing of chemical equation is essential because of the law of conservation of mass, which states that the mass of a system can not be created or removed.
The second equation is balanced
[tex]2KCl_(_a_q_) +Pb(NO_3)_2_(_a_q_) ==> 2KNO_3_(-a_q_) + PbCl_2(_a_q_)[/tex]
This is because the number of elements of each atom in the product side equal the number of elements of each atom on the reactant side.
The first equation is not balanced
[tex]H_2SO_4 _(_a_q) + 2Al_(_s_) ==> Al(SO_4)_3_(_a_q_) + H_2_(_g_)[/tex]
This is because there is 1 molecule of [tex]SO_4^{-2}[/tex] on reactant side as compared to 3 molecules of [tex]SO_4^{-2}[/tex]
To balance the equation we add a coefficient of 3 on sulphuric acid ([tex]H_2SO_4[/tex]) and a coefficient of 3 on hydrogen ([tex]H_2[/tex])
[tex]3H_2SO_4_(_a_q_) + 2Al_(_s_) ==> Al(SO_4)_3_(_a_q_) + 3H_2_(_g_)[/tex]
