Final answer:
The number of atoms of helium in the balloon is approximately 5.06 × 1023 atoms.
Step-by-step explanation:
The number of atoms in a sample of a gas can be calculated using Avogadro's number, which is approximately 6.022 × 1023 atoms/mol. To find the number of atoms of helium in the balloon, we first need to calculate the number of moles of helium using the given density. The density of helium is 1.18 g/L, so the mass of helium in the balloon is 1.18 g/L × 2.85 L = 3.363 g.
The molar mass of helium is 4.003 g/mol, so the number of moles of helium is 3.363 g / 4.003 g/mol = 0.8406 mol. Finally, we can calculate the number of atoms using Avogadro's number: 0.8406 mol × 6.022 × 1023 atoms/mol = 5.06 × 1023 atoms. Therefore, there are approximately 5.06 × 1023 atoms of helium in the balloon.