Question

Why you did not use the 50 ml given in the problem instead you use 1000?

Answer 1

I didn't need it. You can use any volume you wish but if you want to use 50 mL you can. I used 1000 because I've done that for 60 years and I guess it's a habit by now. :-). The answer I have using 1000 mL is 37.04m.

1.42 g/mL x 50 mL = 71.0 g
71 x 0.7 = 49.7 g HNO3 which is 49.7/63 = 0.7889 mols.

The water is 71.0g-49.7 = 21.3 g.
m = mols/kg solvent = 0.7889/0.0213 = 37.04m