How much does a water molecule weigh?
By mx_aman
@mx_aman (2101)
India
4 responses
@stvasile (7306)
• Romania
28 Feb 07
You are wrong!
A molecule of water does not weigh 18g!!!
18 represents the molecular weight of the water molecule, which is not measured in grams. The measuring unity in the case of the molecular weight is "the 12th part of the mass of C12 atom", considered to be the best aproximation for the weight of the hydrogen atom.
They didn't used the mass of the Hydrogen because hydrogen appears in nature as 3 isotopes (Hydrogen, Deuterium and Tritium, that have different masses function to the number of neutrons.
@nairdaleo (104)
• Mexico
5 Mar 07
Yes, hydrogen appears in 3 isotopes, but which one is the ones that combines with water?
The one with the highest percentage in nature, which is the one that has a mass of approximately 1.0079 g/mol. And that's not the reason they're using Carbon-12. That carbon isotope is being used for the sole reason that it's mass is the most exact, 12.000~
To have the most exact amount of particles you then divide the 12 grams into the 12.000 g/mol, and the analysis will lead to 1 mol = 12 grams of Carbon-12
In the same way, we could have chosen any other to make the comparison, but Carbon is the most exact. It has nothing to do with the different isotopes of Hydrogen haha, sorry.
@nairdaleo (104)
• Mexico
5 Mar 07
Well, if the question is right, then the answer is wrong.
The mass of something isn't its weight. Weight is how strong gravity attracts you. In this case it's somewhere about ~176 Newtons.
And it isn't 18 grams either, it is 18 UMAs, which is 18 g/mol. A mol is the amount of elemental particles comparable to the ones contained in a 12 grams sample of the Carbon-12 isotope.
And it's actually ~18.0148 g/mol... haha.
But putting all the previous aside, all you do is calculate the mass of each component of the water molecule, H2O, and add it. One H = 1.0079 UMA; and one O = 15.9998 UMA
However if you want to know the EXACT mass, in grams, of 1 molecule of water (pure), you need to know that in every mol there are 6.023x10^23
( ~602 300 000 000 000 000 000 000 ) molecules of water.
So if you know that 1 mol weighs 18.0148 g/mol, and one mol contains 6.023x10^23 molecules, then 1 molecule will weigh: 2.991x10^(-23) grams
~ 0.0000000000000000000000299100116221 grams
(about ~0.000000000000000000000000006593917906 lb)
2 people like this
@nairdaleo (104)
• Mexico
6 Mar 07
No problem!
Although, I made the same mistake I corrected haha, I forgot to express it in Newtons, The correct approximation is
2.991x10^(-22) Newtons for weight, but I think you were talking anyway about the mass.
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