Converting w/v to mole fraction
You say that you have

10 mL 0.9% w/v NaCl solution

This is an unusual specification and as user M. Farooq pointed out it is ambiguous. So let's assume that this is a reagent that I am to add for some reaction. Is this:
$\quad$(1) 10 ml water (10 grams) and 0.09 grams of NaCl
$\quad$(2) 9.91 ml water (9.91 grams) and 0.09 grams NaCl
$\quad$(3) 10 ml aliquot of 99.1 ml water and 0.9 grams NaCl
Each of these would be slightly different. The gist is that (1) and (2) are different because the densities of the solutions will be slightly different as well as the volumes. (2) and (3) are different because the volumes will vary even though the densities are the same.
Pragmatically such reagent solutions are generally used in a large excess for whatever their use is, so the slight variations don't make any difference in the reaction. However it is just sloppy to give an ambiguous specification when a specific specification could and should be given.
Significant Figures
The other inconsistency with the question posed is with significant figures. The OP has specified 0.9% NaCl, but calculated &quot;mole fractions of 76.43% H2O and 23.56% NaCl&quot;. You can't get four significant figures for the mole fraction when the NaCl concentration only has one significant figure.