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Homework Help: Calulating Atomic Percentages given moles and weights of compounds

  1. Sep 26, 2012 #1
    1. The problem statement, all variables and given/known data
    This is the statement out of a paper:

    Slurry 10 grams of ZnS with 3mL of .2N Cu(CH3COO)2 solution (corresponding to 0.6 atom % Cu) and 1.7 mL of .2N NH4Cl solution (corresponding to 0.34 atom % Cl)

    So my question is how did they calculate the .6 at. % and .34 at. % from the information given

    2. Relevant equations

    The only equations I could find were for mole fractions and converting from weight percent to atomic percent, but by implementing these techniques I was still not able to reproduce the stated percentages.

    3. The attempt at a solution

    I attempted to solve this using excel by calculating wt. percent then converting it to atomic percentages and my numbers are smaller than the ones given.

    I have also done mole fractions and that doesn't seem to be the right way either

    My at. % of Cu was only .145 and for Cl it was .165
    I accounted for all species present

    I will appreciate any guidance for I have been stuck on this for days
    Thank You
  2. jcsd
  3. Sep 27, 2012 #2
    There is a problem with a very strange concept of atoms % that has been used in the paper. It is easy to see where they get their figures from: 3 mL of 0.2 N makes 0.6 meq of copper ion, while 1.7 mL of 0.2 N makes 0.34 mmol of chloride ion that is being slurried with the 10 g of zinc powder.

    But atoms % should mean a ratio of atoms per 100 atoms. 100 atoms of what? there is no clear relationship between mmol and 100 mmol of anything in particular. Ant there is also the problem of use of an outdated and ambiguous concentration unit. Because copper(II) is formally a doubly charged ion, is an equivalent equal to 1 mol or 0.5 mol. If we are looking to reduce copper(II) to metallic copper, it should be 0.5 mol, but reading between the lines I suspect that the recipe you are looking at is for formation of some copper(I) chloro- or ammino- complex, and that we can read meq as mmol.

    The problem does not seem to rest with you, it is with some very strange units and notations in the original recipe.

    In the recipe as you have presented it, we have 0.6 mmol of copper and 0.34 mmol of chloride in a slurry with 10 g of Zn powder -- obviously a vast excess. atom % is a weird fiction, the way the authors have used it.
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