GEN CHEM QUESTION Molecular Formula

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MaxL221

Member
10+ Year Member
Determine the molecular formula of naphthalene consisting of 93.75% C and 6.25% H. (Assume molecular weight of naphthalene, M.W. = 128.16 g)

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

Answer is D, can someone please explain how to get that. I kept getting C.

thanks

gsJ

Junior Member
10+ Year Member
5+ Year Member
There are 3 ways of doing this im showing you 1 very simple way and 1 very long way and the intermediate way.....

Simple easy way I suggest you do on the DAT is look at the molecular weight of 128g thus look at answer D (there are 10 Carbons and 8 hydrogens) so Carbon= 12gx 10 = 120 .... and for hydrogen it is 1gx 8H = 8 .....now add them together and you will get the weight of 128 thus answer 10. (this way is just looking at your answer choices and determining it)

The Long Long Way
Calculating this do as following...consider the percent they give you as 100gram total thus

93.75 gram C/ (12g C/mol) = 7.811 mol C
6.25 g H/(1g H/mol)= 6.25 mol H

now divided by the lowest mole #-----> thus 6.25

7.811/6.25 = 1.25 C
6.25/6.25= 1

now you need to make 1.25 a multiple thus u multiple the 2#s above by 4 giving us
C(1.25x4)= C5
H(1x4)=C4

Now this is the empirical formula and C5H4 has a total mass of 64grams
The molecular formula is given and it is 128gram
Thus divide 128/64 giving a ratio of 2

So just take the empirical formula and multiple by 2
Giving (C5H4)x2= C10H5

Intermediate way

93.75% of C x 128grmas = 120 grams

120/12grams C per mol= 10moles of Carbon

6.25% of H x 128= 8grams

8grams/ 1g H per mol= 8moles of Hydrogen

MaxL221

Member
10+ Year Member
Thanks! that helps!