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Simple chemistry, need help with mol and so forth. Problems ITT.

  1. Nov 19, 2013 #1
    Simple chemistry, need help with "mol" and so forth. Problems ITT.

    Hello, first I'd like to say thank you to this community as I have gotten a lot of help in the physics department before and is basically a big reason of me passing the class.

    Now chemistry is here and I am having a little bit of trouble so I thought I'd put up a few problems here and you can look over my attempt of solving it and maybe help me along the way.


    Problem A) A copper test shows to contain 0.126 mol of copper, the whole test weighs 8.562 g. Does the test contain of 100% copper?

    Attempt to solve: n=m/M

    n= 8.562g / 0.126*6.02*10^23
    n = 8.562 / 7.585*10^22
    n = 4.09*10^25.

    ^--- this is how far I've managed to come on my own. I know the solution is more or less done by now but I am having trouble with what to write, in terms of grams etc to finish off the question. I know that if the answer is not 8.562g the test doesn't contain of 100% copper. As said, don't know how to take it from here.

    Problem B) A sugarcube has the mass 2.1g. The sugar is made of sackaros with the formula C11H22O11

    a) calculate "n" sackaros in the sugarcube.
    b) calculate "n" carbonatoms in the sugarcube.

    Attempt to solve: <---- This is where I am completely lost. I know that I have to use the formula n=m/M and I understand what the formula means. What I don't understand is how to get those numbers from "C11H22O11". I am a complete novice when it comes to this. Any help is very much appreciated.
     
  2. jcsd
  3. Nov 19, 2013 #2

    Borek

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    In A you are doing strange things with Avogadro's number. You don't need it there. You are right about using n=m/M, but you need to solve it for m. What is molar mass of copper?

    In B you need to find molar mass of the sucrose first, that's where the compound formula should come in handy.
     
  4. Nov 19, 2013 #3
    A.

    All you have to do here is find out how many grams 0.126 mol of Cu is. Multiply the molar amount by the MW and you'll end up with 8.007g, which is less than the given 8.562, meaning there must be something else besides the copper.

    B.

    Is that formula correct? Sucrose should be C12H22O11. I'll go through it with the C11 though.

    Using the atomic masses of C, H, and O, you can easily figure out the molecular weight/molar mass (M), which comes out to be 12.011*11+1.0079*22+15.999*11 = 330.2838. You're given m, 2.1g, so all you have to do is plug those into your equation and divide 2.1 by 330.2838. That will give you the amount of moles in 2.1g. Since each mole has 11 C's, just multiply that answer by 11 to get the answer for the second part.
     
  5. Nov 19, 2013 #4

    Okay A) I solved, well you practically solved it but I understand it much more clearly now so thank you for that. As you can see I didn't have much of a handle on that, mainly because I started with chemistry today.

    B) You were right about the formula being C12H22O11, apologies.

    Let's see if I understand this solution. First I have to find out the MW of C, H and O. After that I have to find out the molar mass of each element. Then I have to multiply them (in this case first O and H because "sackaros" contains of H and O?, afterwhich I'll divide 2.1g by the answer to get the ammount of moles in 2.1g? Doesn't that give me the ammount of moles in one gram?) After that is done I will multiply that answer with 12 to get answer for b?
     
  6. Nov 19, 2013 #5

    Borek

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    For B you have to find number of moles (you are close to that - it is a matter of using n=m/M, just plug and chug). Then, multiply number of moles by NA (Avogadro's number) to find number of sucrose molecules (it IS called sucrose, see http://en.wikipedia.org/wiki/Sucrose). Then, you have 12 carbon atoms per each sucrose molecule - how can you calculate how many carbon atoms are there?
     
  7. Nov 19, 2013 #6
    1.00794*22+15.999*12+12.011 = 330.2838

    2.1/330.2838 = 0.00636 moles in 2.1 grams.

    0.00636 * 12 = 0.0763 moles.

    So answer for A is 0.00636 moles and answer for B is 0.0763 moles?

    Is this correct?
     
  8. Nov 20, 2013 #7

    Borek

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    Close, but check your math. Unfortunately that means everything else is wrong (although your method is probably OK now*, just the numbers need correcting).

    No, answer for A is not a number of moles. No idea what you are talking about.

    *Note: your use of "n" vs n confused me and I initially thought B is about calculating number of molecules and number of atoms. I am not so sure now, perhaps you mean just the number of moles.
     
  9. Nov 20, 2013 #8
    Sorry.


    a) Calculate the amount of substance of sucrose in sugar cube.
    b) Calculate the amount of substance of carbon atoms in the sugar cube.

    That is what it said in my document. I am also confused now, I'm sorry for using the chemistry-'language' a bit wrong. I am sure I will learn in time tho.
     
  10. Nov 20, 2013 #9
    What is wrong with me, duno how I can leave out numbers like that.... so

    12.011*12+1.0079*22+15.999*11= 342.295

    2.1/342.295, this is correct because n=m/M right? where m is mass and M is molecular mass and n is moles?

    So, 2.1/342.295 = 0.0064 moles in 2.1g.

    0.0064 * 12 = 0.0768 moles. <--- this is the answer for B. Or I think it is.
     
  11. Nov 20, 2013 #10

    Borek

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    Well, you can always calculate both number of moles AND number of atoms - that would be a good exercise, as you seem to be confused about these things.

    It is worth of noting that number of moles IS number of atoms (or molecules, or any other items). Just like "dozen cars" means 12 of them (so it is a number of items) mole of atoms means 6.02×1023 of them (so it is a number of items again). Mole is just an overgrown dozen, with number of objects selected in such a way mass of the mole in grams equals mass of the object in amu.
     
  12. Nov 20, 2013 #11

    Borek

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    OK so far.

    Again, math is failing you. Or perhaps you should check batteries in the calculator :wink:

    As 0.0064 was wrong, the number you got is not correct (but the method is right). With one important remark: never use rounded down values for calculations. Report them rounded, but do the calculations with full precision, or at least with several (2 or 3) so called guard digits. Otherwise you risk nasty surprises, as rounding errors can be quite large.
     
  13. Nov 20, 2013 #12
    Thank you very much!

    So, 2.1/342.295 = 0.006135

    And for the second part just take 0.006135*12 = 0.07362 to get the number of moles of C's.

    Is it correct to answer for b: "0.07362 moles." or should I say "0.07362 moles of C"?
     
  14. Nov 20, 2013 #13
    I don't understand why that method is right as you said. Because when I take 12.011*12 = 144.132 then take 2.1g/144.132 as per n=m/M I get 0.01457 moles. Which one is correct and why are they different?
     
  15. Nov 20, 2013 #14

    Borek

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    English is my second language, so I am not the best person to ask. The latter (moles of C) is better IMHO (as you have a problem with lack of precision it is better to overdo, than to leave the answer ambiguous).

    Sample doesn't contain 2.1 g of carbon alone!
     
  16. Nov 20, 2013 #15
    Hahahaha, makes a lot of sense. I am so dumb it's sad hahahahahaha.
     
  17. Nov 20, 2013 #16
    Hahahaha, thank you!! I really appreciate all your help.
     
  18. Nov 20, 2013 #17

    Borek

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    You are not dumb, you are learning. Blunders are an important part of the process.
     
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