Question

Suppose I am trying to determine a compound that has 15.87% carbon (C), 2.15 % hydrogen (H), 18.5% nitrogen (N), & 63.41% oxygen (O) (n.b., this does not equal exactly 100%): what is the empirical formula of the compound?

Answer 1

For my piece of mind, I'll add something to each percentage to make the total 100% - 0.02% to the first three, and 0.01% to oxygen.

So, you know that your compound has `"15.89% C"`, `"2.17% H"`, `"18.52% N"`, and `"63.42% O"`. The first step in determining it empirical formula is to divide each element's percentage by its molar mass

`"For C": (15.89%)/(12.0) = 1.34`

`"For H": (2.17%)/(1.00) = 2.17`

`"For N": (18.52%)/(14.0) = 1.32`

`"For O": (63.42%)/(16.0) = 3.96`

The next step is to divide all these numbers by the smallest one; this is done in order to get the ratios of the four elements in the molecule

`"For C": 1.34/1.32 ~= 1.00`

`"For H": 2.17/1.32 = 1.64`

`"For N": 1.32/1.32 = 1.00`

`"For O": 3.96/1.32 = 3`

The empirical formula must only contain subscripts that are integers, which means that your actual empirical formula is

`C_3H_5N_3O_9`