How many moles are present in 4.68 mg of Si (at. Mass of Si = 28 amu)

Molar mass of silicon = 28 u
1 g = 1000 mg 
Given mass of silicon = 4.68 mg = 0.00468 g

$$\begin{gathered} n = \frac{\text{Given mass}}{\text{Molar mass }} \\ n = \frac{0.00468}{28} = \text{0.00017 moles } \end{gathered}$$