What is the empirical formula of the unknown compound?

Question

An unknown compound contains only C , H , and O . Combustion of 3.50 g of this compound produced 8.24 g CO2 and 2.25 g H2O . What is the empirical formula of the unknown compound? Insert subscripts as needed.

Answer 1

An unknown compound contains only C , H , and O . Combustion of 3.50 g of this compound produced 8.24 g CO₂ and 2.25 g H₂O . What is the empirical formula of the unknown compound? Insert subscripts as needed.

The mass of CO₂ will lead you to the amount of C present in the sample, the mass of H₂O will lead you to the amount of H present in the sample, and you can calculate the amount of O by difference.

The subscripts in an empirical formula would represent moles of atoms in a mole of the unknown compound, so you will figure out:

1) The masses of C, H, O present in the sample with the following steps:

a) figure out the mass percent of C in CO₂: ^{mass of 1 mol C} / _{mass of 1 mol CO2} x 100

b) multiply the mass of the CO₂ from the analysis by the mass percent (decimal form, 0.65, rather than

65%) of C in CO₂. That is the mass of C in your 3.50 g sample.

c) figure out the mass percent of H in H₂O as you did in part a).

d) multiply the mass of the H2O from the analysis by the mass percent of H in H₂O (as you did in part b).

This is the mass of H in your 3.50 g sample.

e) Determine the mass of O in your unknown sample by difference:

mass of original sample – (mass of C + mass of H) = mass of O

2) Convert those masses to moles using the molar masses of each element (on periodic table). The empirical formula is the smallest whole-number ratio of the atoms in the compound, such as C₂H₅O, for example. If you arrived at whole numbers when you converted mass to moles, those are the subscripts in your empirical formula. If you instead get decimal values (not whole numbers), go to step 3).

3) Force the smallest of those mole values to be "1" by dividing it by itself, and then divide the other mole values by the exact same number. Your answers should be whole numbers or very close to whole numbers. If one of your values is still a decimal value, such as #.25, or #.33, or #.50 (# represents any number), then multiply all of the mole values by an integer (such as 4, 3, or 2 for the decimal values shown), so all are whole numbers. That is multiply the subscripts in C₂H_4.5O by 2 to get a whole number for H for the proper empirical formula C₄H₉O₂

The empirical formula for your problem is C₃H₄O.