How many atoms are there in 90.2 grams of selenium?

First calculate the moles of selenium by multiplying the mass x molar mass

90.2 * 78.96 = 7122.2 moles

Number of moles multiplied by avogadros number gives number of atoms

7122.2 * 6.022 * 10^{23} = 4.289*10^{27} atoms