trinigirl1401 Posts: 1, Reputation: 1 New Member #1 Sep 17, 2006, 06:34 PM
chemistry moles and mass question
this problem is kicking my butt cause we have multiple choice and I can't seem to get any of the posted answers... please help!

A 0.0100-mol sample of urea, NH2CONH2, contains
4.82 x 1022 atoms.
4.82 x 1024 atoms.
6.02 x 1022 molecules.
4.82 x 1023 atoms.
6.02 x 1023 molecules.

please tell me how to do this as apparently I don't know...
 ilovcali Posts: 206, Reputation: 85 Full Member #2 Sep 19, 2006, 06:22 PM
Hey, I'm a chem grad student. Are you a frosh?

Anyway, so what you need to realize is that urea is a molecule. Then use Avogadro's Number. That is 6.02 x 10^23 anything/mol. So the conversion is as follows:

0.0100 mol urea X 6.02 x 10e23 molecules urea/1 mol urea The moles will cancel, and you'll be left with molecules of urea.
 J_9 Posts: 40,298, Reputation: 5646 Expert #3 Sep 19, 2006, 07:03 PM
But Cali, isn't there an easier way to figure out A's number?

For the beginner? If I remember correctly, I might be wrong, but there is an easier formula for A's number, that will get you to your expected outcome.
 kp2171 Posts: 5,318, Reputation: 1612 Uber Member #4 Sep 19, 2006, 09:04 PM
Originally Posted by ilovcali
Hey, I'm a chem grad student. Are you a frosh?

Anyway, so what you need to realize is that urea is a molecule. Then use Avogadro's Number. That is 6.02 x 10^23 anything/mol. So the conversion is as follows:

0.0100 mol urea X 6.02 x 10e23 molecules urea/1 mol urea The moles will cancel, and you'll be left with molecules of urea.
ilovecali - while your answer is a start, you did not solve the problem and you assumed too much. Just because urea is a molecule doesn't mean the answer needs to be in molecules.

0.01 * 6.02*10^23 gives 6.02*10^21 molecules of urea. This is not an answer that is available. This rules out the answers with "molecules" as the unit.

that leaves only answers with atoms. Since urea has 8 atoms per molecule, you then need to take the number of molecules times the number of atoms.

6.02*10^21 * 8 = 48.16 * 10^21 which is 4.82*10^22 atoms.

the key is to do this step by step. Moles to molecules first, then molecules to atoms if you still are looking for a matching answer.

j_9... I don't think shortcuts are beneficial to those doing these kinds of problems in most circumstances, as being able to address multiple cancellations of units is a skill that most gen chem students need to master without tricks or shortcuts.

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