The analysis of an organic compound showed that it contains 0.175 mol C, 0.140 mol H, and 0.035 mol N. Its molecular is about 160 g/mol. What is the subscript on in the chemical formula?

1 Answer
anor277
Sep 2, 2016

molecular formula=C10H8N2

Explanation:

We divide thru the molar quantities by the SMALLEST such molar quantity, that of nitrogen:

C:0.175⋅mol0.035⋅mol = 5

H:0.140⋅mol0.035⋅mol = 4

N:0.035⋅mol0.035⋅mol = 1.

And thus we get an empirical formula of C5H4N.

But the molecular formula is always a whole number mulitple of the empirical formula:

(molecular formula)=n×(empirical formula)

And then solve for n.

160⋅g⋅mol−1 = n×(5×12.011+4×1.0076+14.01)⋅g⋅mol−1.
By this calculation, the empirical mass is 78⋅g⋅mol−1. This is tolerably close to half the measured molecular mass. Thus n≅2. This often occurs if the molecular mass has to be estimated by more traditional means than mass spectroscopy, i.e. boiling point elevation, or by the isopiestic method.

Thus molecular formula=C10H8N2

Related questions
See all questions in Determining Formula
Impact of this question
6251 views around the world
You can reuse this answer
Creative Commons License