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Help with 2 gen chem questions please

  1. Jan 22, 2014 #1
    Hello everyone, first time poster here.
    I'm hoping someone can help me with these two questions, they're part of a study guide for my upcoming chem 1 exam.

    Mythical element Z has an avg atomic mass of 107.9 u and is known to have only two naturally occurring isotopes. If 48.16% of Z exists as 109Z (108.971 u) what's the other isotope and its atomic mass?

    a) 107 Z ; 106.9 u
    b) 108 Z ; 107.9 u
    c) 110 Z ; 109.9 u
    d) 111 Z ; 111.1 u
    e) 110 Z ; 110.2 u

    and this one:

    Fluorine can be precipitated out of solution with the use of calcium to create CaF2 (s). The use of 85.75mL of 0.3510 M Ca(NO3)2 precipitated out all of the fluorine present in a 4.585g sample of a soluble metal fluoride dissolved in water. Calculate the weight percent of fluorine in the sample.

    a) 24.95%
    b) 11.44%
    c) 56.48%
    d) 10.10%
    e) 12.58%

    I attempted both a few times but with no success, I'm just not sure how to start for either, especially the wording in the second problem is very confusing for me.

    Any help is super appreciated, thank you so much!
  2. jcsd
  3. Jan 23, 2014 #2
    Hello Amar314,

    The questions are actually simple. For the first one, the questions says that Z's average atomic mass is 107.9u and 48.16% of Z exists as 109Z (108.971 u); which means, 100-48.16 = 51.84% is made up of the other one.

    So, going by the formula for Mean/Average, we get

    Average Mass = (P')*[Mass of one] + (P'')*[Mass of the other]

    2) For the second question,

    Ca(NO3)2 + MFx (x=unknown) ------> CaF2

    (Hint:No. of equivalents of Ca+2 = Number of equivalents of F- )

    Try to find out the N factor.

    Equivalents = Moles*Nfactor

    NB: If you are not aware of equivalent weight and stuff, I suggest you to read :

    http://www4.ncsu.edu/~franzen/public_html/CH201/lecture/Lecture_19.pdf [Broken]

    Hope this helps!

    Last edited by a moderator: May 6, 2017
  4. Jan 23, 2014 #3
    Thank you so much! I really appreciate your help!!

    Last edited by a moderator: May 6, 2017
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