hello piyush 

This is a case of limiting reagent, limiting reagents are those reagents which  control the amount of product formed by depleting earlier. 

from reaction of HCL and NaOH we found the HCL left was =0.01 moles

total HCL provided was=0.02 moles

so HCL consumed was =0.01 moles

so moles of CaCO3 used= 0.005 , which is half the no of moles of CaCO3 provided.

so % purity =50%

hope it helps