# Mass of ions with respect to amu

1. Aug 8, 2012

### johnny_b_good

Dear all,

I have a question concerning a basic chemistry topic: atomic mass units. The current amu system defines 1 amu = 1/12 of a Carbon-12 isotope (about 1.66 * 10^-27 kg). Therefore, every other atom is assigned a amu value based upon its relationship to Carbon-12.

My question concerns ions. Given the following elementary reaction,

Na+ + Cl- ⇔ NaCl

Any general chemistry student can calculate mass of Na+ required to produce one mole of NaCl. However, when calculating the value for Na+ we calculate the value using 22.99 g/mol. Why is that? I understand that this value is a weighted average of relative abundances of Na isoptopes. But why do we ignore the mass of the electron? Is it due to the fact that an electron weighs about 1/1836 the mass of a proton? If this is true, then isn't this assumption limited (for as you increase the number of moles, the number of atoms increases in relation to avogadros number.... which would eventually make the mass of the one less electron a non=ignorable factor). Thanks for the help!!

2. Aug 8, 2012

### pa5tabear

You've got the right idea. If you wanted to calculate more precisely, you could try and find more precise data, with more decimal places. An electron weighs 0.000544662 amu, so you could subtract that amount from your value. Most of the time this will be insignificant.

If you're looking at millions of moles, this value will be larger, but your error percentage will remain the same.