So,
First of all, remember that a "mol" has two meanings:
In this case, we want to find the number of atoms in 15.2g of Fluorine. Using the fact that the molar mass of F is 18.998g/mol, we could write:
[tex]15.2gF\cdot\frac{1\text{molF}}{18.998gF}\cdot\frac{6.022\cdot10^{23}atomsF}{1\text{molF}}=4.818\cdot10^{23}atomsF[/tex]
