Answer
There are $1.2368\times10^{18}$ formula units of NaCl in 0.12 mg.
Work Step by Step
Using the molar mass of the compound provided, we can simply convert from milligrams to grams to moles and then from moles to formula units or molecules.
First to convert from miligrams to grams, we simply divide by 1000 so; $0.12 mg\div1000 = 1.2\times10^{-4} g$
$1.2\times10^{-4} g NaCl\times\frac{1 mole NaCl}{58.44 g/mol} \times\frac{6.023\times10^{23} formula units}{1 mole NaCl}
= 1.2368\times10^{18} formula units of NaCl$.
