Calculating amount of molecules

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Blanca Cervantes
Posts: 17
Joined: Wed Nov 21, 2018 12:18 am

Calculating amount of molecules

Postby Blanca Cervantes » Thu Jun 27, 2019 12:58 pm

Can anybody explain how they went about solving the following problem?:

Calculate the amount of urea molecules, OC(NH2)2, in 2.3 x 10^5g of urea, which is used in facial creams and, on a bigger scale, as an agricultural fertilizer.

The correct answer is 3.8 x 10^3, but I keep getting a different number. I solved this problem by first dividing the total mass of the sample by the molar mass and then multiplying my result by Avogadro's constant.

My steps:
Finding molar mass:
m(O) + m(C) + m2(N) + m4(H) = 60.055

Dividing mass sample by molar mass:
(2.3 x 10^5)/ 60.05526= 3829.80

Multiplying the amount (in moles) by Avogadro's constant
N= 3829.80 x 6.022x10^23= 2.306 x 10^27

George_Zhu
Posts: 29
Joined: Mon Jun 17, 2019 7:23 am

Re: Calculating amount of molecules

Postby George_Zhu » Fri Jun 28, 2019 10:47 am

Comparing with the correct answer and your answer, the goal of this question is to calculate amount of the substance in unit mol, instead of the number of the particle.

Kevin To 1B
Posts: 32
Joined: Sat Feb 02, 2019 12:15 am

Re: Calculating amount of molecules

Postby Kevin To 1B » Fri Jun 28, 2019 2:06 pm

Blanca Cervantes wrote:Dividing mass sample by molar mass:
(2.3 x 10^5)/ 60.05526= 3829.80


That right there is 3.8 x10^3 moles, as indicated by your answer. Is there any chance the question asks for the amount in moles instead of molecules? (as a typo)

Blanca Cervantes
Posts: 17
Joined: Wed Nov 21, 2018 12:18 am

Re: Calculating amount of molecules

Postby Blanca Cervantes » Sun Jun 30, 2019 7:29 pm

There most likely was a typo in the textbook question


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