Chemical analysis shows that citric acid contains 37.51% C, 4.20% H, and 58.29% O. What is the empirical formula for citric acid?

1 Answer
Jan 12, 2017

Answer:

#C_6H_8O_7#

Explanation:

As with all empirical formula problems of this kind, it is useful to assume that there #100*g# of compound, and we work out the atomic ratios on this basis.

#"Moles of carbon"# #=# #(37.51*g)/(12.011*g*mol^-1)=3.12*mol#

#"Moles of hydrogen"# #=# #(4.20*g)/(1.00794*g*mol^-1)=4.17*mol#

#"Moles of oxygen"# #=# #(58.29*g)/(15.999*g*mol^-1)=3.64*mol#.

We divide thru by the SMALLEST molar quantity, and then normalize the result (i.e. convert it to whole numbers!):

#CH_(1.34)O_1.17#

To normalize, we mulitply this formula by a factor of #6# to give
#C_6H_8O_7#. And there we have it: the empirical formula, which is the simplest WHOLE number ratio defining constituent atoms in a species.

We could find out from mass spectroscopy that citric acid gives a molecular ion at #192*"amu"#. Can you tell me the MOLECULAR formula of #"citric acid"#? Remember the molecular formula is always a multiple of the empirical formula, and the mulitple MAY be #1#.