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### Topic: Basic Problem-Moles-Molecules  (Read 3072 times)

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##### Basic Problem-Moles-Molecules
« on: November 18, 2009, 01:47:37 PM »
I need to find the number of water molecules in ${ 2.56 mL }$ of ${ H_2O }$

The density of water at 4 degrees C is given as ${ d=\frac{1g}{mL} }$ therefore ${ 2.56 mL H_2O }$ contains ${ 2.56g H_2O }$ at that temperature.

${ (2.56g H_2O)(\frac{1 mol H_2O}{18g H_2O}) \approx 0.142 mol H_2O }$

If there is anything wrong with the latex, I will be editing it.

I'm not sure how to relate the number of molecules to the number of moles. I know how to find the number of atoms of any element. So could I just find the number of hydrogen atoms in 1 mole of H_2O }[/img] and then divide that number by 2? Or I guess it would just be easier to find the number of oxygen atoms in 1 mole. 1 water molecule contains 1 oxygen atom.

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##### Re: Basic Problem-Moles-Molecules
« Reply #1 on: November 18, 2009, 01:54:30 PM »
Actually, I think it's simplier than I though. The number ${ N_A }$ doesn't seem to apply exclusively to atoms. So I should be able to use the conversion factor ${ \frac{N_A molecules}{1 mol H_2O} }$  right?

#### cth

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##### Re: Basic Problem-Moles-Molecules
« Reply #2 on: November 18, 2009, 03:56:03 PM »
Yes, simply multiply by NA the mole number you have to get the number of molecules.

One mole of something (it could be anything like atoms, molecules, cars, stars,...) is always defined as containing NA objects. Just like a dozen is defined as containing 12 objects.