Basic Stoichio with a missing element? Please help :(!?

a compound is composed of element X and hydrogen. Analysis shows the compound to be 80% X by mass with three times as many hydrogen atoms as X atoms per molecule. Which element is element X?


the answer is carbon

2 Answers

  • Ashley
    Lv 6
    8 years ago
    Favorite Answer

    XH₃ means a mass of 3. so 3 is 20% of some number. 300/20 = 15, so the whole mass is 15. That means the mass of X is 12. That would be carbon

  • Anonymous
    8 years ago

    So your formula is XH3, right?

    It's got to be a group 3 or 5 element, because those take 3 bonds. NH3, ammonia, BH3 boron trihydride or AlH3, aluminum trihydride.

    N is 14g/mol. H 1g/mol

    NH3 is 17g/mol.

    14/17 = 0.82 (82% mass) while 3/17 = 0.18 (18% mass)


    B 10.8g/mol so BH3 = 13.8g/mol

    10.8/13.8 = 78.3% B, 11.7% H


    Al 27g/mol so AlH3 30g/mol

    27/30 = 90% Al, 10% H

    Can't be this one, % too high

    Gotta be N or B... which is a choice on your test? Pick that one.

    Close enough for govt work! :)

    Hope this helps! :)

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