How many grams are in 6.78 x 10^24 atoms of He?

45.05g

Explanation:

Based on the Avogadro's hypothesis, we understood that 1 mole of any substance contains 6.02x10^23 atoms.

From the above findings,

1 mole of He also contains 6.02x10^23 atoms.

1 mole of He = 4g

If 4g of He contains 6.02x10^23 atoms,

then Xg of He will contain 6.78x10^24 atoms i.e

Xg of He = (4x6.78x10^24)/6.02x10^23

Xg of He = 45.05g

Therefore, 45.05g of He contains 6.78x10^24 atoms