All you really have to do here is to find a conversion factor that will allow you to go from grams of selenium to moles of selenium
The elements' molar mass is added at the bottom of the element box. In this case, selenium is said to have a molar mass of
You need to go from grams to moles, so set up your conversion factor as
#"1 mole Se"/"78.96 g"#
Now simply multiply the value given to you by the conversion factor to get the number of moles of selenium present in your sample
#22.5 color(red)(cancel(color(black)("g"))) * "1 mole Se"/(78.96color(red)(cancel(color(black)("g")))) = color(darkgreen)(ul(color(black)("0.285 moles Se")))#
The answer is rounded to three sig figs, the number of sig figs you have for the mass of selenium.