[ibstudent]

How can you know that number of moles of atoms in a compound?

For example, in 2 moles of H2O, how many moles are there for hydrogen and how many moles for oxygen?

I would really appreciate it if someone can show me the way. 

Thanks in advance

[DevaDevil]

a mole is a number of molecules; 6.022 x 10²³ molecules, in fact.
It was invented to make exactly what you ask more convenient:

two moles of a certain molecule consists of two times the number of atoms per molecule in moles of each atom in its compound.

so for two moles of water:

oxygen atoms: 2 x 1 = 2 moles.

hydrogen atoms: 2 x 2 = 4 moles.

Do not forget though that hydrogen and oxygen molecules consist of 2 atoms, so 1 mole of oxygen atoms is 1/2 mole of oxygen molecules, and 2 moles of hydrogen atoms = 1 mole of hydrogen molecules.