0.1 mol of NaCl is dissolved in 100g of water.
The mole fraction of NaCl is

Given, no. of moles of solute(n_s)=0.1mol

Weight of water=100g

no. of moles of solvent(n_o)=$\frac{100}{18}=5.55$

${\mathrm{X}}_{\mathrm{NaCl}}=\frac{{\mathrm{n}}_{\mathrm{s}}}{{\mathrm{n}}_{\mathrm{s}}+{\mathrm{n}}_{\mathrm{o}}}=\frac{0.1}{5.55+0.1}=0.0177$