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