3) 1 O2 +1 H2 --> 1 H2O2
4) 2 Al + 3 H2(SO4) --> 1 Al2(SO4)3 + 3 H2
5) 2 K3(PO4) + 3 MgCl2 --> 1 Mg(PO4)2 + 6KCl
6) 2K + 1 Br2 --> 2 KBr
7) 3 Hg(OH)2 + 2 H3(PO4) --> 1 Hg3(PO4)2 + 6 H(OH)
8) 3 Ca + 2 NH3 --> 3 CaH2 + 1 N2
9) 1 Na2(CO3) + 1 Ca(OH)2 --> 2 Na(OH) + 1 Ca(CO3)
10) 4 Al + 3 O2 --> 2 Al2O3
Basically, with these questions, you want both sides to have the same number for each of the atoms. With these problems, you count the number of each element for the reactants (before the arrow) and the number of each element for the products (after the arrow) and try to get them to equal each other using the least amount of numbers.