4. Balance the following equations (show work)
a.___H2(g) +___O2(g) →___H2O(1)
b.____Fe(s) +___O2(g)→___Fe2O3(s)
c.___Al(s) +___HCl(aq) →___AlCl3(aq) +___H2(g)
d.____CaCO3(s) +____HCl(aq) →____CaCl2(aq) +____CO2(g) +____H2O(1)
