[chelbyk]

Following an example in class, I attempted to complete a homework problem. I got an answer but it is a very different number from what the example was. If anybody could take a look at this and perhaps see my error it would be greatly appreciated.

PROBLEM: What mass of glycine, C2H5NO2, contains 8.61 x 10^24 atoms of oxygen?

ANSWER:
1. First I found the MOLECULAR WEIGHT= 75g/mol
2. I took 8.63 x 10^24 oxygen atoms x 1 molecule/2 oxygen atoms x 1 mole/ 6.02 x 10^23 molecules x 75.0 g
3. I then calculated and got the ANSWER: 1.67 x 10^23 grams.

However, in class the example worked out to a whole and I was wondering if I made a mistake or was correct. Any help please, thanks.

[Padfoot]

First convert atoms to mol.  Your amount of glycine will be half this (since 1 molecule of glycine contains 2 atoms of O) and the mass can be found by multiplying by molar mass.

As for where you went wrong, hard to say, I can't really follow your working - what is it you were trying to do in step 2??

[chelbyk]

step 2 is the process we were taught to arrive in which you go:

Oxygen Atoms (8.63 x 10^24) to molecules by taking the ratio (1 molecule divided by 2 oxygen atoms).

Multiply this then by taking 1 mole and dividing it by avogadros # (1 mole divided by 6.02 x 10^23 molecules) and then multiplying that by atomic weight over on mole (75.0 grams divided by 1 mol)

its a step that goes from atoms to molecules to moles to grams.

[Padfoot]

Ok...  On second thoughts then your working does seem to make sense.  Assuming you punched in the numbers correctly you should be right.

My way was simpler though 

[enahs]

There are various ways to work this.


8.61 x 10²⁴ oxygen atoms * (1 molecule glycine ) * (1 mol  )                           *   (75 g) = 536g
                                     (2 oxygen atoms     )    ( 6.022e23 molecule glycine)      (1 mol)


Or, you know you need 8.61 x 10²⁴ oxygen atoms.
This corresponds to 14.3 mols of oxygen atom, right? You know ever molecule of glycine provides 2 molecules of oxygen, right? So you need 7.15 mols of glycine, agree? 7.15 mol x 75 g/mol = 536, correct?

You need to recalculate and include sigfigs.