Na+ = 70 mg mL−1
WNa in 50 mL solution = 70 × 50 mg = 3500 mg = 3.5 g
Moles of Na+ in 50 mL solution =3.523
Moles of NaNO3 = moles of Na+ = 3.523 mol
Mass of NaNO3=3.523×85=12.94 g Mass of NaNO3=13 g