Answer
$Co_{3}As_{2}O_{8}$
Work Step by Step
Convert the mass of each constituent element into moles by dividing by its molar mass:
$0.672g\ Co\times\frac{1mol}{58.933g}=0.0114mol\ Co$
$0.569g\ As\times\frac{1mol}{74.922g}=0.00759mol\ As$
$0.486g\ O\times\frac{1mol}{16.00g}=0.0304mol\ O$
Formulas have whole number subscripts so divide all the amounts by the smallest and round if the number is really close. Then multiply by whatever factor makes them all a whole number or very close to a whole number.
$\frac{0.0114mol\ Co}{0.00759}=1.5\times2=3$
$\frac{0.00759mol\ As}{0.00759}=1\times2=2$
$\frac{0.0304mol\ O}{0.00759}=4\times2=8$
So the empirical formula for this compound is:
$Co_{3}As_{2}O_{8}$
