# Number of atoms in CaSO4

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.

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## Answers and Replies

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

The 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.

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.

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!

Mentor
I got:

molar mass - 172.171 g/mol

moles - 1.940×10-5

oxygen atoms - 7.011×1019