[sega18]

Hello everyone,

First post here. Nice to meet you all. I was going through the third chapter of my textbook and I noticed a little blurb in the 3rd chapter about relative masses. It mentioned that a mass spectrometer gives not a mass (in g), but rather a ratio of masses, and from this information, we derive what we know about Chemistry. It also mentioned that Carbon-12 is defined as having a mass of 12 amu, and that every other atom's mass is calculated using that definition.

This ENTIRE concept confuses the crap out of me. If there exists a guide of some sort online about this, I would be ecstatic if someone could point me to it. Otherwise, an explanation?

Thanks in advance,

Segev

[UG]

A mass spectrometer operates by determining the charge-to-mass ratio (m/z) for gas phase ions. For an ion with charge z = 1, the m/z value corresponds to the molecular mass, M, of the ion. If z = 2, the recorded mass of the ion is half its actual mass, and so on.

[Arkcon]

It mentioned that a mass spectrometer gives not a mass (in g), but rather a ratio of masses, and from this information, we derive what we know about Chemistry. It also mentioned that Carbon-12 is defined as having a mass of 12 amu, and that every other atom's mass is calculated using that definition.

To clarify it a little more for you -- although those two facts are close to each other, they don't describe the same concept.  What UG: said answers your question.  The bit about carbon = 12 as the standard by which other elements are compared is a fundamental fact, with or without mass specs.

[sega18]

Thanks UG and Arkcon. Why is Carbon-12 = 12 amu the standard? What makes it special? Why not say Hydrogen-1 = 1 or Neon-2 = 2 or element-X = X?

[Arkcon]

We actually used to use hydrogen=1, it was switched to carbon=12.  I don't really know why.