The mass of 11.45 mol Arsenic atoms is 857.9 grams.
From the periodic table of the elements, we can see that the atomic weight of Arsenic, As, is 74.92160 amu. We can make a direct conversion to the molar mass of As as 74.92160 g/mol. This gives us two conversion factors for the mass per mole of As:
To convert the given moles of of an element to mass, we multiply the given moles times the conversion factor that cancels moles and leaves grams.