How many atoms are in a sample of 72.8 grams calcium?

First find the moles

moles of the calcium = 72.8 / 40

moles of calcium in 72.8g = 1.82mol

number of atoms = moles x Avogadro constant

atoms = 1.82 x 6.02x10+23

so the number of atoms is 1.096x10+24