Question

Give the empirical formula of each of the following compounds if a sample contains the following quantities of each element.

(a) 0.0091 mol C, 0.0186 mol H, and 0.0045 mol O

Please show me how to do this. My notes are very confusing and I don't know how to apply them to this problem.

Answer 1

you want a mole ratio. Take the smallest number, .0045, divide it into each of the other moles...

O 1
C 2
H 4

C2H4O is the empirical formula, and I would even on this guess structure as
CH3OH

Answer 2

Okay, thanks. Do I always round up to the nearest whole number?

Answer 3

Yes, the ratio is always in whole number; however, one must be careful not to throw away too much. For example if the number comes out to be 2.05 it can be rounded down to 2.0 but if the number is 2.5 that can't be rounded to either 2.0 or 3.0. if the ratio was 1.0 to 2.5 that's the same as 2.0 to 5.0. The ratio of 1 to 3.33 is in whole numbers 3 to 9.99 which would be rounded to 3.0 to 10.0. I usually feel relatively secure in rounding numbers up to 2.1 or 3.1 or something like that but even 2.25 I don't like to round since 1 to 2.25 is the same as 4.0 to 9.0