if 0.5 moles of H2SO4 combine with 0.2 moles of CaSO4 then how many moles of Ca(OH)2 will be there?
This is a limiting reagent problem. You know that because amounts for BOTH reactants are given.
I work these problems, the long way, by solving TWO stoichiometry problems. Here is a worked example of how to do those problems.
http://www.jiskha.com/science/chemistry/stoichiometry.html
Using 0.5 mole H2SO4, calculate moles Ca(OH)2 formed.
Using 0.2 mole CaSO4, calculate moles Ca(OH)2 formed.
In limiting reagent problems the smaller value is ALWAYS the correct one to choose (and the reagent producing the smaller value is called the limiting reagent).