1g of hydrogen contain 6 * 10 23 atoms.the atomic weight of helium is 4u .it follows that the number of atoms in 1g of he is;

Since atomic weight of H is 1 u, 1 g means 1 mole.
1 mole of Hydrogen contains 6*1023 atoms.
No of moles in 1 g of He = 1/4 = 0.25 mol
No of atoms in one mole = ‚Äč 6*1023 
No of atoms in 0.25 moles = 0.25*6*1023 = 1.5*1023 atoms

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