Question

How many moles of `"H"_2"O"` contain `2.60 * 10^23` molecules of water?

The answer is 0.432 mole of H2O but I dont know how to get that answer. I dont know how to set up the problem

Answer 1

`0.432`mol

Explanation:

Divide the given number of molecules by Avogadro's constant. Avogradro's constant is the number of atoms/molecules in a mole of a substance.

Therefore,

`(2.60 * 10^23 \ "molecules")/(6.022 * 10^23 \ "molecules/mol") = "0.432175 moles" ~~ "0.432 moles"`

Answer 2

Well, how molecules are present in ONE MOLE of water...?

Explanation:

By definition, there are `6.022xx10^23` such molecules, or `N_A` such molecules in ONE mole of water. And thus in such a quantity there are `N_A` oxygen atoms, and `2xxN_A` hydrogen atoms...and the mass associated with this numerical quantity of water molecules is approx. `18*g`...

And so we simply take the quotient....

`"Moles of water"=(2.60xx10^23*"water molecules")/(6.022xx10^23*"water molecules"*mol^-1)`

`=0.432*mol`...and thus a mass of `0.432*molxx18.01*g*mol^-1=7.78*g`...

Instead of the mole, we could use the dozen, or the gross, or some other numerical quantity ... it just happens that one mole of hydrogen ATOMS have a mass of `1*g` more or less precisely.. which is why we use this absurdly large quantity .. Capisce?