Question

Combustion analysis of 17.471 g of trioxane produces 10.477 g of water and 25.612 g of carbon dioxide. The molar mass of trioxane is approximately 90. g/mol. Determine the molecular formula of this compound.

Answer 1

First determine the grams of carbon in 25.612 g CO₂ and the grams of hydrogen in 10.477 g H₂O.

carbon: 25.612 g x (12.011 g / 44.0098 g) = 6.9899 g

hydrogen: 10.477 g x (2.0158 g / 18.0152 g) = 1.1723 g

Determine the grams of oxygen in the sample by subtraction.

17.471 - (6.9899 g + 1.1723) = 9.3088 g

Convert grams of C, H and O to their respective amount of moles.

carbon: 6.9899 g / 12.011 g / mol = 0.58 mol

hydrogen: 1.1723 g / 1.0079 g/mol = 1.16 mol

oxygen: 9.3088 g / 15.9994 g/mol = 0.58 mol

Divide each molar amount by the lowest value, seeking to modify the molar amounts into small, whole numbers.

carbon: 0.58 mol / 0.58 mol = 1

hydrogen: 1.16 mol / 0.58 mol = 2

oxygen: 0.58 mol / 0.58 mol = 1

the empirical formula of the substance is CH₂O

Molecular formula:

the weight of empirical formula CH₂O is 30

The molar mass of trioxane is approximately 90. g/mol

90 / 30 = 3

Molecular formula= (CH₂O)₃ = C₃H₆O₃