Question

compare the weight of 1 molecule of oxygen & 1 atom of hydrogen.

Answer 1

1 molecule of oxygen contain 2 atoms of oxygen,one atom of oxygen weight 16amu and 1 molecule weight 5.31*10^-23grams (since O2 is two Oxygens that are each 16 grams per mole
so O2 is 32 grams per mole
a mole is 6.023 x 10^23 molecules
So the mass of 1 molecule of O2 is 32/(6.023 x 10^23) grams
I assume you can use a calculator. It should be 5.3 x 10^-23 grams.

If you like, this is also 53 yoctograms. (This is 53 x 10^-24 grams.)

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1 atom of hydrogen contain only one atom of hydrogen and it weight 1.099 amu