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I don't get relative atomic mass Watch

1. I understand the formula.

Two isotopes for Chlorine, 37 and 35.

Apparently 75% of Chlorine isotopes are Cl 35 and 25% are 37.

(75 x 35) plus (25 x 37) is 3550 divided by 100 is 35.5.

1. It says in a sample of 100 Cl atoms, 75% will have a mass of 35. Well surely that will depend entirely on the sample taken. One sample in one part of the world will vary massively to another sample elsewhere in the world, so why do we just assume that 75% will always be Cl35?

2. Why do we not just add the atomic mass of all known isotopes then divide by how many there are, in this case you would get Cl 36. This to me makes more sense because we cannot possibly know an accurate % of which are 35 and which are 37 given the size of the planet and inaccuracy of taking samples (100 atoms of Cl is nothing compared to total number on Earth, even if repeated many times).

3. How can you work out RAM of an element yourself if you're not given abundance data? (If thats possible, i'm still very new to chemistry)

Thanks
2. When I did physics A-level, the questions said to assume that, on Earth, the acceleration due to gravity was 10m/s^2. Had I done a different maths A-level, they'd have said it was 9.81m/s^2.

The latter is closer to the truth, but it's still not 'right'. Amongst other things, it varies according to where you are. But the physics questions were much more about you understanding the basic physics than being able to do multiplication to so many significant figures.

Here, there are going to be so many chlorine atoms that the proportion is going to be 'about' 3:1. It's trying to get you to understand the basic chemistry - elements have isotopes, and they're in different proportions according to the element - not do some more complicated maths.

For #3, you could say that it's probably near some figure if you know which are stable, but you can't.
3. (Original post by AccessToUni25)
I understand the formula.

Two isotopes for Chlorine, 37 and 35.

Apparently 75% of Chlorine isotopes are Cl 35 and 25% are 37.

(75 x 35) plus (25 x 37) is 3550 divided by 100 is 35.5.

1. It says in a sample of 100 Cl atoms, 75% will have a mass of 35. Well surely that will depend entirely on the sample taken. One sample in one part of the world will vary massively to another sample elsewhere in the world, so why do we just assume that 75% will always be Cl35?
There is going to be a lot of atoms in any sample you take. Therefore any sample obtained without selecting for a specific isotope will have a near identical proportion of isotopes. Radioactivity complicates this for very old radioactive samples (as the Ar will change over time), but then these can be used for applications such as radio dating.

2. Why do we not just add the atomic mass of all known isotopes then divide by how many there are, in this case you would get Cl 36. This to me makes more sense because we cannot possibly know an accurate % of which are 35 and which are 37 given the size of the planet and inaccuracy of taking samples (100 atoms of Cl is nothing compared to total number on Earth, even if repeated many times).
Yes we do know the proportions to a very high accuracy. In fact the Ar of Cl is known to be 35.453 u ± 0.002, so we know it to within 0.002.

3. How can you work out RAM of an element yourself if you're not given abundance data? (If thats possible, i'm still very new to chemistry)

Thanks
You can't. If you were given a sample you could run it through a mass spec though.

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Updated: September 15, 2016
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