Molten NaCl is crystallised with $2.0\times 10^{-3}$ mol per cent of $\mathrm{SrCl}{}_2$ as impurity (mol of $\mathrm{SrCl}{}_2$ per 100 mol of NaCl). Each $\mathrm{Sr}{}^{2+}$ replaces two $\mathrm{Na}{}^{+}$ ions by occupying one site and leaving one cation vacancy. Using $N_A=6.022\times 10^{23}\mathrm{mo}{\mathrm{l}}^{-1}$, find the number of cation vacancies per mole of NaCl in units of $10^{18}$ (nearest integer).

Numerical answer type, answer in x 10^18, tolerance 0.5.