An unknown compound contains only C , H , and O . Combustion of 5.60 g of this compound produced 13.7 g CO2 and 5.60 g H2O . What is the empirical formula of the unknown compound?

Answer 1

To find the empirical formula, we want to first find the moles of each element.

moles C = 13.7 g CO2 x 1 mol CO2/44 g x 1 mol C/mol CO2 = 0.3114 moles C

moles H = 5.60 g H2O x 1 mol H2O/18 g x 2 mol H/mol H2O = 0.6222 moles H

To find moles O, we must convert moles of C and H to grams and subtract that from original mass (5.60 g) and then convert to moles, as follows:

mass of C = 0.3114 mol C x 12 g/mol = 3.74 g C

mass of H = 0.6222 mol H x 1 g/mol = 0.622 g H

mass of O = 5.60 g - 3.74 g - 0.622 g = 1.238 g O

moles O = 1.238 g O x 1 mol/16 g = 0.0774 moles O

Now that we have moles of each element, we want to find the lowest whole number equivalent, so we divide all by the lowest value of 0.0774 to obtain:

C = 0.3114/0.0774 = 4

H = 0.6222/0.0774 = 8

O = 0.0774/0.0774 = 1

So empirical formula is C₄H₈O