Complete the balance equations for each of the following reaction:
a. __NaBr + __Ca(OH)2 → __CaBr2 + __NaOH
b. __NH3 + __H2SO4 → __(NH4)2SO4
c. __Pb + __H3PO4 → __H2 + __Pb3(PO4)2
d. __AlCl3 + __Na2SO4 → __Al2(SO4)3 + __NaCl