# What is the relationship between the amu and the gram?

1. May 6, 2006

### IB

The textbook told me that there's a relationship between the amu and the gram, but I fail to see any. Could you please show me? Thank you.

2. May 6, 2006

### dav2008

They are both are a measure of mass.

1 atomic mass unit = 1.66053886 × 10-27 kilograms = 1.66053886 × 10-24 grams. You can look this value up in books/online pages.

You can derive it by realizing that 1 amu is defined as 1/12th the mass of a C-12 atom.

Last edited: May 6, 2006
3. May 6, 2006

### IB

But how is 1g of anything = 6.022 X 10^23 amu

Isn't 1 amu = 1/12 Carbon-12 so therefore 6.022 X 10^23 amu = 6.022 X 10^23 amu / 12 Carbon-12 ?

I'm quite confused. Where did chemists get 1g of anything = 6.022 X 10^23 amu.

4. May 6, 2006

### dav2008

A lot of things are defined in respect to very specific situations. For example, the second is defined as "9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom". The meter is defined as "the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second".

That doesn't mean that we are restricted to using seconds only when talking about cesium atoms, or using meters only when talking about how far light travels.

The amu is just a convenient unit to use when talking about things with mass on the scale of atoms. (And those things are typically atoms)

Last edited: May 6, 2006
5. May 6, 2006

### GCT

It's somewhat convenient, the relation between grams to amu that is, in the sense that the value in amu of a particular atom/compound is pretty much the same with the mass of a mole of that atom/compound. Thus reading up on the history of moles, and why it was created, should add some excitement to this matter, that of the relationship between the mass of an individual atom/compound with that of a sample. The whole thing was somewhat devised, that is they were seeking for such a method of gravimetric analysis, and that's why the "mole" is so special.