A salt crystal has a mass of 0.15 mg. How many NaCl formula units does it contain?

2.56*10^-6 moles

Explanation:

Number of formula units is the same as the number if moles of NaCl present.

Number of moles = mass / molar mass

Molar mass of NaCl = (23+35.5) = 58.5gmol-1

Given mass of NaCl = 0.15*10^-3g

Number of moles = 0.15*10^-3/58.5

Number of moles = 2.56*10^-6 moles