DAT Achiever - Test 3 #51 GC

[thejamespark]

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question on a problem that is super easy but I do not know why I can't seem to understand it...
DAT Achiever Test 3 #51.
Determine the molecula formula of napthalene consisting of 93.75% C and 6.25% H. (Assume molecular weight of napthalene M.W. = 128.16 g)

A)C4H3
B)C5H4
C) C8H6
D)C10H8
E)C12H6

Answer is D: C10H8.

I chose answer C doing the math:
93.75 / 12 ~ 8 C
6.25 / 1 ~ 6 H

I just dont understand or what I am overlooking that this answer is C10H8 and not C8H6...
Any help would be appreciated...

 

[xtra333]

empirical formula *x= molecular formula
from calculations.
93.75/12= 7.8 ish
6.25/1= 6
since 6 is the least, you choose C4H3 * x= 128.16
48+3=51
128.16/51=2.51
2.51*C4H3 = (around the molecular mass of) C10H7.5
round the number of H up to get C10H8.

hope that helps.

 

[Blue87Fiero]

You aren't taking into account the molecular weight they gave you? 93.75% of napthalene is carbon and 6.25% is Hydrogen. Multiply the 128 by .9375 to get grams of carbon then divide by molecular weight of carbon. Do the same for hydrogen

 

[Blue87Fiero]

You are trying to solve as though it was question that stated what is the empirical formula of a sample that contains X% of carbon Y% of hydrogen and Z% of oxygen. Then you could assume a size for you sample was anything, 100 grams works best because then the % can equal grams. Divide by molecular weight of the molecules and you good.

 

[thejamespark]

ahhhh i see what ur saying. thanks.
such an idiot -_- thanks guys..