# Chemistry exam question

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#1
Hi. I answered this exam question, and I managed to get the correct answer. However, I am just wondering if my method was correct and the reason behind it was also correct.

The question was: An isotope of sodium has a relative isotopic mass of 23.0. Calculate, to three significant figures, the mass, in grams, of a single atom of this isotope of sodium (the Avogadro constant is 6.02 x 10^23 mol^-1).

I knew that molar mass was mass per mole. For sodium, it was 23.0. Avogadro's constant = 6.02x10^23 which is number of atoms per mole.

I thought to find the mass, I would turn Avogadro's constant into 1 since I only want 1 atom, so I divide by 6.02x10^23 and I would do the same to molar mass of sodium to get 23/6.02x10^23.

I'm not sure if i've just made things complicated, but if there's a simpler reason behind dividing 23/6.02x10^23 then please explain. Thank you.
0
4 years ago
#2
If the molar mass is 23.0 per mole, then we can say that:

n=m/Mr

1= m/23

=> m = 23 grams

This means that the mass of 6.02*10^23 atoms of sodium is 23 grams.

So to find one atom, you can just use ratios:

6.02*10^23 atoms : 23 grams

=> 1 atom : 23/(6.02*10^23) grams
1
#3
(Original post by Welshstig)
If the molar mass is 23.0 per mole, then we can say that:

n=m/Mr

1= m/23

=> m = 23 grams

This means that the mass of 6.02*10^23 atoms of sodium is 23 grams.

So to find one atom, you can just use ratios:

6.02*10^23 atoms : 23 grams

=> 1 atom : 23/(6.02*10^23) grams
Is that sort of what I did then? Thanks for the reply.
0
4 years ago
#4
(Original post by Xenice)
Is that sort of what I did then? Thanks for the reply.
Exactly what you did!
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