I am revisiting the concept of mole and I decided to figure out random values (for instance: mass of 1 molecule of water) to see how much I understand the concept of moles and how it relates to amu. So here is how I started:

There is 1 mole of C atoms in 12g of C and 1 C atom is 12 amu. Therefore, there is 1 mole of 1 amu in 1g. That means there are 6.02e23 nucleons in 1 gram. So, 1 nucleon is 1g/6.02e23= 1.66e-24g. A molecule of water has 14 nucleons (6 neutrons and 6 protons from oxygen and 2 protons from both H); thus, 1.66e23 x 14 should be the g of a water molecule, which turns out to be 2.33e-23. However, this is not the correct mass of water molecule (2.992 x 10¯23 grams).


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  • 2
    $\begingroup$ Who said oxygen contains 6 neutrons and 6 protons? $\endgroup$ – Ivan Neretin Jan 31 '18 at 7:09
  • $\begingroup$ 8............ wow. I had the valence electrons number in mind.Thanks $\endgroup$ – Singh Jan 31 '18 at 7:19
  • $\begingroup$ Note that the atomic mass unit (amu) is deprecated, and instead the newer definition should be used, which is the unified atomic mass unit (u). Also note, that the molar masses of atoms and compound reflect their relative abundance, too. There are different isotopes and it is generally not true to say one carbon atom has the mass 12 u (and especially not 12 amu). $\endgroup$ – Martin - マーチン Jan 31 '18 at 8:20

As Ivan pointed out in the comments, I had the molar mass of water incorrect. By fixing the molar mass to 18g/mol, I get the correct answer.


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