Serotonin (molar mass 176 g/mol) is a compound that conducts nerve impulses in brain and muscle. It contains 68.2% C, 6.86% H, 15.9% N and 9.08% O. What is the molecular formula?

1 Answer
anor277
Feb 16, 2017

#"Serotonin"-=C_10H_12N_2O#

Explanation:

As with all these problems, we ASSUME #100*g# of compound, and we work out the percentages of each element on a molar basis:

#"Moles of carbon"# #=# #(68.2*g)/(12.011*g*mol^-1)=5.68*mol#.

#"Moles of hydrogen"# #=# #(6.86*g)/(1.00794*g*mol^-1)=6.81*mol#.

#"Moles of nitrogen"# #=# #(15.9*g)/(14.01*g*mol^-1)=1.14*mol#.

#"Moles of oxygen"# #=# #(9.08*g)/(15.999*g*mol^-1)=0.568*mol#.

Note that normally the analyst would not give you #%O#; you would be expected to work it out by difference.

And now we divide thru by the SMALLEST molar quantity, that of oxygen, to give an empirical formula of:

#C_10H_12N_2O#.

And this is the empirical formula, the simplest whole number ratio defining constituent atoms in a species.

Now the molecular formula is ALWAYS a multiple of the empirical formula. That is #"Molecular formula"-="(empirical formula)"_n#.

Thus the molecular mass must show this proportionality:

#(C_10H_12N_2O)_n=176*g*mol^-1#

So #nxx(10xx12.011+12xx1.00794+2xx14.01+15.999)*g*mol^-1=176*g*mol^-1#

Clearly, #n=1#, and the #"empirical formula"# is the same as the #"molecular formula"#.

This approach is standard, however, most of the time you would not be quoted a percentage oxygen composition. You would be expected to calculate this yourself by difference.

Related questions
See all questions in Empirical and Molecular Formulas
Impact of this question
14376 views around the world
You can reuse this answer
Creative Commons License