How to know whether answer is in mols or atoms

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Talia Leano 3H
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How to know whether answer is in mols or atoms

Postby Talia Leano 3H » Mon Oct 12, 2020 11:42 am

The following is an example problem from the textbook: "Self-test E.5A: Calculate the amount of urea molecules, OC ( NH 2 ) 2 , in 2.3 × 10^5 g of urea, which is used in facial creams and, on a bigger scale, as an agricultural fertilizer."

The correct answer given was in moles. How do you know to end at moles for this question and not multiply that by avagadro's constant to get the exact number of molecules since 1 mol of molecules = 6.0221x10^23 molecules?

Also if someone could give me an example of a problem wording that would require you to turn the moles of molecules into exact molecule count that would be super helpful!

Megan Sparrow 1G
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Re: How to know whether answer is in mols or atoms

Postby Megan Sparrow 1G » Mon Oct 12, 2020 11:45 am

I was having the same issue. I believe that questions were asking for the number of molecules in terms of moles (a measurement of the atoms or molecules in a give data set). I believe the tests will be clearer on the wording. Also, I'm not sure if this helps but usually when trying to find exact amount of atoms or molecules, the question will ask for formula units.

Nika Gladkov 3b
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Re: How to know whether answer is in mols or atoms

Postby Nika Gladkov 3b » Mon Oct 12, 2020 11:47 am

I believe that, since the question asks for "amount", it implies that the question is asking for mol as moles are a measure of the amount of molecules. However, if the question asks for number of molecules/atoms, you would need to multiply by Avogradro's constant to find that number from the moles you obtained.

For example, if the problem was "Calculate the number of urea molecules, OC ( NH 2 ) 2 , in 2.3 × 10^5 g of urea, which is used in facial creams and, on a bigger scale, as an agricultural fertilizer" or "Calculate the number of nitrogen atoms, OC ( NH 2 ) 2 , in 2.3 × 10^5 g of urea", you would need to convert moles to number of FU with Avogadro's constant.

Hope that helps!

Crystal Yu 3C
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Re: How to know whether answer is in mols or atoms

Postby Crystal Yu 3C » Mon Oct 12, 2020 11:47 am

I agree that the wording of this problem is confusing. I think because it asks for the "amount of __ molecules", the word amount is referring to moles. If the question instead asked you to calculate the "number of molecules of ___", then you would use Avogadro's number to solve for the molecules of the substance. I hope this helps! :)

Samudrala_Vaishnavi 3A
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Re: How to know whether answer is in mols or atoms

Postby Samudrala_Vaishnavi 3A » Tue Oct 13, 2020 7:55 am

Firstly, the "amount" always refers to the moles of something, so you know your answer will not have to be multiplied by Avogadro's number to cancel out the moles for atoms/molecules/formula units which are the same things by the way. The amount never refers to atoms so multiplying moles times atoms/ mole will leave you with atoms, which is not what we want.

Carly_Lipschitz_3L
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Re: How to know whether answer is in mols or atoms

Postby Carly_Lipschitz_3L » Sun Oct 25, 2020 3:30 pm

I think that if it had asked for the number of molecules you would need to multiply by Avogadro's constant. Since it asked for the amount of molecules I think that is an indicator to answer in moles.


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