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Homework Help: Molecular Formuals from Empirical Formula Help Please!

  1. Jan 21, 2010 #1
    1. The problem statement, all variables and given/known data
    An unknown compound contains 21.8 g of phosphorus and 28.2 g of oxygen. It's molar mass is 284 g/mol. what are its empirical and molecular formulas?

    Elements and the Weight:
    P = 31 g/mol
    O = 16 g/mol

    Given Measurements:
    21.8g P, 28.2g O, MM = 284g/mol



    2. The attempt at a solution

    This is my math:
    Elements and the Weight:
    P = 31 g/mol
    O = 16 g/mol

    Given Measurements:
    21.8g P, 28.2g O, MM = 284g/mol

    Converting the given measurements to moles:
    21.8gP / 31 = .70 mol P
    28.2gO / 16 = 1.8 mol O

    Getting the ratio:
    .70 / .70 = 1
    1.8 / .70 = 2.57 (rounds up to 3)

    Ratio:
    1:3

    Empirical Formula : PO3

    Formula Mass of PO3:
    79g/mol

    Finding the common factor:
    284(the given weight)/ 79(the weight of PO3) = 3.594 (rounds up to 4)

    Writing the Molecular Formula:
    4(PO3) = P4O12

    Getting the Molar Mass of the Molecular Formula:
    P4 = 124
    O12 = 192
    P4O12 = 316
    316 =/= 284(is what it should be)
     
  2. jcsd
  3. Jan 21, 2010 #2
    Nevermind, I did some research and found out that if the ratio is ~2.5 that you're supposed to multiply it by 2 to get even numbers.
     
  4. Jan 21, 2010 #3
    ====================
    You got a ratio of 2.57:1. instead of rounding to 3, try 5:2, which is a lot closer.
    Then your formula is P2O5 or P4O10. This makes sense since P and O have oxidation numbers of +5 and -2, so the electrons balance out to zero.
     
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