Answer: The number of moles of water in the solution is 0.868 mole.
Explanation:
The summation of mole fractions of different components in solution is equal 1.0.
If a solution composed of different components (a, b, c, d, ...., n) and each component has its mole fraction (Xa, Xb, Xc, Xd, ......,Xn) then Xa + Xb + Xc + Xd + ... + Xn = 1.0
For the mentioned NaCl solution; the solution composed of NaCl and water: XNaCl + XH2O = 1.0
Also, it is important to define the mole fraction: XNaCl = nNaCl/ntotal and ntotal = nNaCl + nwater; where n id the number of mole (n = mass/molar mass)