Question

How many moles of water, H2O, contain 2.0×1022 molecules of water? (See the hints for assistance in interpreting scientific notation.)

Express the quantity in moles to two significant figures.

Answer 1

You have to use Avogadro's number which is 6.02x10^23. The units of Avogadro's number is molecules/mole so you divide the number of molecules by Avogadro's number which will give you the number of moles.
(2.0x10^22 molecules)/(6.02x10^23 molecules/mole)=0.033 moles