The molar mass of salt (NaCl) is 22.99+35.45 = 58.44 g/mol. If we have 0.18 mg of salt, this is equivalent to 0.00308 mmol NaCl, or 3.1E-6 mol NaCl. By definition, 1 mole contains 6.02E23 formula units, so multiplying this by the number of moles gives the answer, which is: 1.9E18 formula units.
