# Number of atoms in CaSO4

1. Sep 29, 2013

### nando94

I had a multiple choice problem on my pop quiz. How many oxygen atoms are there in .00334 grams of CaS04 X 2H2O.

The molar mass is 172 g/mol. So .00334/172 = 1.9 x 10-5 moles x 6 since there are 6 oxygen x 6.022 x 1023 = 7.016 x 1019.

None of the answer choices had that. Did I do it wrong? I'm gonna talk to my teacher about it tomorrow but its been bothering me lol.

Last edited: Sep 29, 2013
2. Sep 29, 2013

### epenguin

Some small arithmetical mistake? What is the nearest to yours among the answers? Yours seems to be close, I get 6.865 X 1019.

Oddly I get a slightly different molar, 175.5 mass from yours (miscounted H?) but anyway 1.9 X 10-5 moles. Your reasoning is correct so in that case, say it was an multiple choice exam you should go to the nearest to your number.

3. Sep 29, 2013

### nando94

I think the slightly different molar mass is due to differences between periodic tables.

The answer choices were 2, 6, 5.41 x 10^22, 5.81 x10^22, 1.11 x 10^21. Since your answer was very close to mine, I'm thinking that there was a typo on the quiz. If I saw your answer among the ones above I would have picked it but they weren't even close. Hopefully it is a typo haha and thanks for the help!

4. Sep 30, 2013

### Staff: Mentor

I got:

molar mass - 172.171 g/mol

moles - 1.940×10-5

oxygen atoms - 7.011×1019