Answer :

pyromorphite
CrO₃ << we've a 3 oxygen atoms

1 mole of CrO₃ = 100g
in 1 oxygen atoms we've a 6,022*10²³ atoms (Avogadro constant)

in 100g of CrO₃-----------------3*6,022*10²³ oxygen atoms
in x g of CrO₃ ------------------ 4,5*10²³ oxygen atoms
x = 24,91g of CrO₃

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