Doping of NaCl with 10−3 mol % SrCl2 means that 100 moles of NaCl are doped with 10−3 mol of SrCl2
∴1 mole of NaCl is doped with SrCl2=10−3100mole=10−5 mole
As each Sr+2 ion creates one cation vacancy, therefore concentration of cation
vacancies =10−5 mol/mol of NaCl =10−5×6.023×1023=6.023×1018mol–1