Question

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?

Answer 1

`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`.