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Old Aug 31st 2017, 06:41 AM   #1
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Finding unknown element

Hi folks! A chem student here. I have a homework problem that I feel might be a curve ball and thought i should get a second opinion. Here it goes....

​A mass of 0.6807g of carbon and a mass of 3.453g of an unknown element contain equal number of atoms.Find the identity of the unknown element?
I am thinking that if the unknown element is 5.072 times heavier than carbon than, all that I have to do is find out which element has a mass of 60.91.

My method: 3.453g / 0.6807g = 5.072
5.072 * 12.01 = 60.91

I believe the element is Cobalt. Am i correct?
Thanks for your replies!
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Old Sep 1st 2017, 07:50 PM   #2
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That's right ... were looking for an atomic weight of 60.91 assuming the carbon was
the mix of isotopes we find on Earth giving an adv.wt.or 12.01
.... it could be pure carbon 14, or have more 14 than usual .... in which case the unknown could be a range of elements...

cobalt (27) on Earth has an average at wt of 58.93 ... the next element , nickel (28) goes down in at wt to 58.693 !!!....copper(29) up again to 63.546

So no element fits exactly ( assuming all elements are taken from Earth) ....Cobalt is 1.98 out ....copper 2.636 out ....

So cobalt is the most likely , next most likely is nickel .... but it could be copper .

Last edited by oz93666; Sep 1st 2017 at 07:53 PM.
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