<p>$M_i$ is the molar mass of the <em>i<sup>th</sup></em> element. Dividing $w_i$ by $M_i$ yields the number of moles of that element in the sample.</p>

<p>There are two primary ways of looking at abundances. One is by weight (or mass), the other is by number of atoms. Consider beryllium and uranium. Their abundances are very similar when looked at in terms of mass: 1.9 parts per million (ppm) for beryllium versus 1.8 ppm for uranium. In terms of molar abundance, they aren't so similar. In this sense, beryllium's abundance is 4.3 ppm; for uranium, it's only 0.15 ppm.</p>

CC BY-SA 3.0
2014-11-07T11:27:27.057Z
Obviously. Beats me as to why he used such obscure terminology and not the usual moles and molar weight/mass.
Gimelist
CC BY-SA 3.0
2014-11-07T11:33:02.300Z
I've never run across the term "gram formula unit" before, but apparently that term is a long-winded synonym for "mole", particularly in geochemistry.
David Hammen
CC BY-SA 3.0
2014-11-07T17:03:03.720Z
Apparently where? I'm a geochemist (albeit a young one) and I've yet to encounter it.
Gimelist
CC BY-SA 3.0
2014-11-07T17:38:31.167Z
"Mole" as a unit is fairly new. It was added to the International System of Units (aka metric system) in 1971. Before that scientists used things such as atomic weight (which is still used), gram-molecules, gram-atoms, and gram-formula. The article [Milton, "A new definition for the mole based on the Avogadro constant: a journey from physics to chemistry." *Philosophical Transactions of the Royal Society A* 369.1953:3993-4003 (2011)](http://rsta.royalsocietypublishing.org/content/369/1953/3993.long) discusses the past, present, and future of assessing quantity of matter.
David Hammen
CC BY-SA 3.0
2014-11-07T09:09:04.997Z
2014-11-07T09:09:04.997Z
David Hammen
<p>I'm reading Shaw's "Trace Elements in Magmas" and in his page 16 he writes the following:</p>

<blockquote>
  <p>...It is necessary to look at the relationship between $c_i$ and
  $x_i$, i.e. the weight and molar concentrations. In any phase of
  weight $W$ the weight of component $i$ is $w_i$ and if the formula
  weight of $i$ is $M_i$, then the number of gfu (gram-formula unit
  concentration) of $i$ is $n_i$, where $n_i = \frac{w_i}{M_i}$...</p>
</blockquote>

<p>What does he mean when he mentions the "formula weight" ($M_i$) and the gfu (gram formula unit)?</p>

<geochemistry>
What are the formula weight and gram-formula unit (gfu)?
Gimelist
# What are the formula weight and gram-formula unit (gfu)?

(Asked by: username_0 on 2014-11-05)

I'm reading Shaw's "Trace Elements in Magmas" and in his page 16 he writes the following:

> ...It is necessary to look at the relationship between $c\_i$ and
> $x\_i$, i.e. the weight and molar concentrations. In any phase of
> weight $W$ the weight of component $i$ is $w\_i$ and if the formula
> weight of $i$ is $M\_i$, then the number of gfu (gram-formula unit
> concentration) of $i$ is $n\_i$, where $n\_i = \frac{w\_i}{M\_i}$...

What does he mean when he mentions the "formula weight" ($M\_i$) and the gfu (gram formula unit)?

**Tags:** geochemistry


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## 1 answers

### Answer by username_1 on 2014-11-07. Score: 6 | Accepted answer

$M\_i$ is the molar mass of the *ith* element. Dividing $w\_i$ by $M\_i$ yields the number of moles of that element in the sample.

There are two primary ways of looking at abundances. One is by weight (or mass), the other is by number of atoms. Consider beryllium and uranium. Their abundances are very similar when looked at in terms of mass: 1.9 parts per million (ppm) for beryllium versus 1.8 ppm for uranium. In terms of molar abundance, they aren't so similar. In this sense, beryllium's abundance is 4.3 ppm; for uranium, it's only 0.15 ppm.

> **Comments:**
> **username_0**: Obviously. Beats me as to why he used such obscure terminology and not the usual moles and molar weight/mass.  
> **username_1**: I've never run across the term "gram formula unit" before, but apparently that term is a long-winded synonym for "mole", particularly in geochemistry.  
> **username_0**: Apparently where? I'm a geochemist (albeit a young one) and I've yet to encounter it.  
> **username_1**: "Mole" as a unit is fairly new. It was added to the International System of Units (aka metric system) in 1971. Before that scientists used things such as atomic weight (which is still used), gram-molecules, gram-atoms, and gram-formula. The article [Milton, "A new definition for the mole based on the Avogadro constant: a journey from physics to chemistry." *Philosophical Transactions of the Royal Society A* 369.1953:3993-4003 (2011)](http://rsta.royalsocietypublishing.org/content/369/1953/3993.long) discusses the past, present, and future of assessing quantity of matter.  

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