How many atoms of hydrogen and oxygen are in hydrogen peroxide?

Interesting question. Balanced equation.

H2 + O2 >> H2O2 ( let's find moles and limiting reactant )

10 O2 molecules (1 mole/6.022 X 10 ^23) = 1.66 X 10^-23 moles O2

same for H2, so one to one and reaction is driven by above moles, but use O2 for convenience.

actually, as all is one to one, you get back 10 molecules H2O2 anyway

1.66 X 10^-23 X 6.022 X 10^23 = 9.99, or 10 molecules of H2O2