How many grams NaCl should be weighed out to prepare 1 L of a 100-mg/L solution of (a) Na+ and (b) Cl−? the answer is (a) 0.254 g/L but how?

(a)
Molar mass of Na⁺ = 23.0 g/mol
Molar mass of Cl⁻ = 35.5 g/mol
Molar mass of NaCl = (23.0 + 35.5) g/mol = 58.5 g/mol

Assume that 1 L of solution is prepared.
Mass of NaCl that contains 100 mg of Na⁺ = (100 mg) × (58.5/23.0) = 254 mg = 0.254 g
Mass of NaCl required = 0.254 g/L

(b)
Assume that 1 L of solution is prepared.
Mass of NaCl that contains 100 mg of Cl⁻ = (100 mg) × (58.5/35.5) = 165 mg = 0.165 g
Mass of NaCl required = 0.165 g/L