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Home Q & A Board Science-Related Homework Help Chemistry (Moderator: Laser_3)
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tommygirl5012 tommygirl5012
wrote...
11 years ago
An Organic compound contains 80% by mass of carbon and 20% by mass of hydrogen. (RAM: C=12, H=1). If the relative fomular mass of the compound is 30, what is its molecular formula?
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juiceboxhero Author
wrote...
#1
11 years ago
Assume 100 g sample-
80 g C / 12 g/mol = 6.67 mol
20 g H / 1 g/mol = 20 mol
ratio of C : H = 1 : 3
empirical formula = CH3 (molar mass = 15)
Since the molecule has a molar mass of 30, the molecular formula is 2 times the empirical formula-
C2H6
rj957 Author
wrote...
#2
11 years ago
First find the empirical formula, or the smallest whole number ratio of the elements.  Assume 100 grams; you can then use 80 grams of C and 20 of H.  Turn each into moles by dividing by molar mass.  80/12.01 and 20/1.01.  Divide each of the two answers by the smallest, which should be C.  That will give you the empirical formula.  Get the molecular formula by determining the molar mass of the empirical formula and dividing the molecular formula mass by that number.  Then multiply each subscript by that number.
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