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Identify 2 Unknown Elements in a Chemical Equation?

  1. Nov 2, 2017 #1
    1. The problem statement, all variables and given/known data
    A silly sample of a metallic element M, reacts completely with 0.0237 mols of X2 to form 6.78 g MX. What are the identities of M and X?

    2. Relevant equations

    mols = grams/molar mass

    3. The attempt at a solution
    (kinda have no clue what to do because i'm not sure what to do. There aren't any elements to compare the unknown with !)

    Here's the equation i got

    M + X2 > MX
    balancing it: 2M + X2 > 2MX

    6.78g MX * (???)

    I honestly don't know what to start with and would appreciate if someone gave me hints ! :(
  2. jcsd
  3. Nov 3, 2017 #2

    As long as the reaction is complete and you know the moles of one of the substances, you automatically know the moles of the other substances.

    The mass of one of them is given, so you can calculate its molar mass. This can lead you to the solution (search in periodic table).
  4. Nov 3, 2017 #3
    I get that MX has a molar mass of 143 g/mol but that's not for separate elements!

    Actually, i just figured out that i was given 5.11 grams of X?

    Would M be Ag? I found its molar mass to be 107.8g/mol

    I also found X to be Ga because i found its molar mass is 70.46 g/mol

    (Im pretty sure i messed up! .-. I'll post my work when i wake up~ tomorrow xD)
    Last edited by a moderator: Nov 4, 2017
  5. Nov 4, 2017 #4


    Staff: Mentor

    That helps.
    How did you get these values?
  6. Nov 4, 2017 #5
    I was actually given 5.11 grams of M
  7. Nov 4, 2017 #6
    0.0237 mols X2 * (2 mol MX / 1 mol X2)
    =0.0484 mol MX

    6.78g MX/0.0484 mol Mx =143 g/mol

    0.0237 mol x2 * (2 mol M/2 mol X) = ?? Mols M

    5 .11g M / ?? Mols M = 107.8 g/mol
  8. Nov 4, 2017 #7


    Staff: Mentor

    Okay, that makes more sense.
    I agree with silver then.

    The molar mass of X2 that you probably calculated earlier is fine as well - what does that imply for an atom of X?
  9. Nov 4, 2017 #8
    You calculated the molar mass of MX to be 143 g/moland the molar mass of M to be 107.8 g/mol.
    So you can calculate the molar mass of X directly from these data.

    Beware: It is 0.0474 and not 0.0484 mols MX.
  10. Nov 4, 2017 #9


    User Avatar

    Staff: Mentor

    Beware: X2.
  11. Nov 4, 2017 #10

    I like Serena

    User Avatar
    Homework Helper

    That must be why the problem statement said: 'A silly sample of a metallic element M.'
    silly = silvery :oldeyes:
  12. Nov 4, 2017 #11
    Actually, i ready 5.11 g as Silly ! XD Look~
  13. Nov 4, 2017 #12


    User Avatar

    Staff: Mentor

    Funny, as 5.11 g is not necessary to solve the problem.
  14. Nov 4, 2017 #13
    Since there is 5.11 g of M and 6.78 g of MX in the products, can i subtract the two to get grams of X2?

    So i can do 0.0237 mols X2 * (2moles X / 1 mole X2) = 0.0474 moles X

    6.78 g - 5.11 g = 1.67 g X2

    1.67 g X2 / (.0237 moles X2) = 70.46 g/mols X2 /2 = 35.23 g/mol of X = Chlorine (35.453 g/mol)
  15. Nov 4, 2017 #14
    How do you use just the molar masses to figure out X?
  16. Nov 4, 2017 #15

    As long as MW(MX)=143 and MW(M)=107.8, you can use the equation MW(MX)=MW(M)+MW(X). The only unknown is MW(X).

    From the MW (molecular weight) of X, you can define the chemical element X directly.
  17. Nov 4, 2017 #16
    Oh ! ! that makes sense! Thanks! This confirms my answer too!
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