How many molecules are in 40 grams of water?

To do this, we will do some simple conversions. 

First, find the amount of grams in one mol of H2O.

Per hydrogen atom, there will be about 1 gram. Per oxygen molecule, there will be about 16 grams.

1 x 2 + 16 x 1 = 18 grams per mole.

Then we will find home many moles we have.

40/18 = 20/9 moles

Because there are 6.022 E23  (Avogadro's number) molecules in one mole, we will multiply the amount of moles times this number.

20/9 x 6.022 E23 = 1.34 E24 molecules

The final answer is 1.34 E24, or 1.34 x 10^24 molecules.
Hope I helped!

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