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Homework Help: How to find the molecular formula out of the molar mass?

  1. Sep 27, 2008 #1
    Here is the problem. I have a series of questions that I don't know how to do. They are all basically the same so I will just give an example of one and if I could get a point the right direction that would be great!

    The molar mass of an important industrial solvent is 78.0 g and its empirical formula is CH. What is its molecular formula?

    Anther slightly related question that might be answered as part of the first question is

    How many moles are in 35 g of potassium chloride?
  2. jcsd
  3. Sep 28, 2008 #2


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    Staff: Mentor

    What is molar mass of the hypothetic CH group? How many such groups are needed for the molecule to have molar mass of 78?
  4. Sep 29, 2008 #3
    As Borek said,

    (CxHy)n = 78.0 (You are given 1 x and 1 y)

    1) Find the mass of the hypothetical group
    here : x (mass of carbon = 12) + y (mass of hydrogen = 1) = 13

    2) Find how many possible hypothetical groups you can have for 78.0 g
    here : n = 78.0 / 13.0 = 6,

    3) Just multiply replace x,y and n and you will get the actual formula
    here : (C1H1)6 = C6H6

    ... and you found Benzene :)

    The second part is as simple. Just use a periodic table to find the mass of K and Cl, thus finding the mass of 1 mole of KCl, then just divide the mass given in the question:

    1 mole = (35.5 + 39.1) 74.6 g
    35 g = ( 35 / 74.6 ) 0.47 moles
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