The equation converts the unit mol/kg to mol/g, so it is necessary to include the relation between kg and grams. The relation is that 1kg is equal to 1000g.
So, we have to fill in the missing part with 1kg in the numerator and 1000g in the denominator (In this way the kg are canceled):
![18\frac{mol}{\operatorname{kg}}.\frac{1\operatorname{kg}}{1000g}=0.018(mol)/(g)]()
Finally, the result is 0.018 mol/g.