Answer:
1,024 moles/liter
Explanation:
MNaCl: 58,44
nNaCl: 15g/58,44 g/mole = 0,256 mole
When NaCl dissolves in water it ionizes. After ionization of 1 mole of NaCl, in the 1 mole of Na+ and 1 mole of Cl- ions will be present in the medium.
NaCl (s) => Na+(aq) + Cl-(aq)
For this case 0,256 moles of Na+ ion will be present in the medium.
Concentration of Na+ = 0,256 moles / 0,250 L of water = 1,024 moles/liter