Empirical Formula

Question

An unknown compound contains only C, H, and O. Combustion of  3.60 g of this compound produced 8.48 g CO2 and 2.31 g H2O. What is the empirical formula of the unknown compound? Insert subscripts as needed.

Answer 1

To find the formula, we need to find moles of C, H and O. We can do this from the grams as follows:

8.48 g CO2 x 1 mol CO2/44 g x 1 mol C / mol CO2 = 0.193 mol C

2.31 g H2O x 1 mol H2O/18 g x 2 mol H / mol H2O = 0.257 mol H

To find mols O, we convert mols C and H to grams and subtract that from 3.60 g to get grams of O, then convert that to mols of O....

0.193 mol C x 12 g /mol = 2.32 g C

0.257 mol H x 1 g/mol = 0.257 g H

g O = 3.60 g - 2.32 g - 0.257 g = 1.02 g O

1.2 g O x 1 mol O / 16 g = 0.0639 mols O

Summary of moles:

C = 0.193 mols

H = 0.257 mols

O = 0.0639 mols

Divide all by 0.0639 in order to get whole numbers and we find...

3 mols C

4 mols H

1 mol O

Empirical formula = C₃H₄O

Answer 2

Try to figure out the combustion formula.

To do this find a ratio between the number of moles of CO₂, H₂O, and O₂.

We know that 8.48g of CO₂ is produced. Change this into moles.

0.102

Change grams of H₂O into moles

.128

Next get the grams of O₂. We know that the mass of the reactants equals the mass of the products

7.19

Change grams of O₂ into moles

.225

Scale up the mole ratios to be whole numbers

CO₂:H₂O:O₂ = 0.102:0.125:0.225 = 4:5:9

Therefore, our chemical reaction looks something like

X + 4O₂ --> 5H₂O + 9CO₂

Right now, we have 0 carbon on the left and 9 carbon on the right, so we need 9 more carbon on the left

We have 0 hydrogen on the left and 10 on the right, so we need 10 more hydrogen

We have 8 oxygen on the left and 23 oxygen on the right, so we need 15 more oxygen

So the empirical formula for X would be C₉H₁₀O₁₅