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Homework Help: Finding element X in an unknown compound

  1. Jun 9, 2015 #1
    1. The problem statement, all variables and given/known data
    An unknown compound weighs 1.023 kg. It is known that element X composes 28.3% of the compound. Given that the molar mass is 240.5 g/mol, find element X.

    2. Relevant equations
    mole = mass / atomic weight
    (n = m / Ar)

    mole = mass / molecular weight
    (n = m / Mr)

    mole = number of particles / Avogadro's number
    (n = P / NA)

    3. The attempt at a solution
    I tried solving using the first equation, but I don't know them mass, Ar or mole of element X. I solved for the mole of the substance using the second equation, but I don't know how to find the unknown element from there. I don't think the last equation is applicable in this particular case, but I included it just in case.
  2. jcsd
  3. Jun 9, 2015 #2


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    Staff: Mentor

    Neither of the equations you listed is applicable here, answer is much more trivial.

    Pretty basic math: you are being told x grams compose 28.3% of 240.5 g. How many grams is x?

    Not that I know what the final answer is. Unless you did some mistake copying numbers, there is no element that fits.
  4. Jun 9, 2015 #3
    I think I've worked out the problem. 28.3% of 240.5 g isn't actually giving the Ar of the element - it's giving the Ar of the element x the number of said element in the compound. I can't work out how I could find just the Ar on its own, without the number of the element factoring into the problem.
  5. Jun 9, 2015 #4


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    Staff: Mentor

    The only valid approach is to guess - assume there is a single atom, or two atoms, or three atoms per molecule and so on. Sadly, none of the numbers you will get gives a reasonable answer.
  6. Jun 9, 2015 #5


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    Homework Helper
    Gold Member

    And then the first sentence of all makes no sense, and on any near guess as to sense it is irrelevant... I don't think we should try to guess further.
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