Answer
386.667 grams/mole
Work Step by Step
Molar mass has units of grams/mole. If we have that 0.00105 moles has a mass of 0.406 grams, we just set up a ratio as follows:
$\frac{0.406g}{0.00105 mole}= \frac{x grams}{1 mole}$
Then we cross multiply and find that:
0.406 = 0.00105x and therefore x = 386.667, which translates to 386.667 g/mol
