Question

8.0 g of a certain Compound X, known to be made of carbon, hydrogen and perhaps oxygen, and to have a molecular molar mass of 44./gmol, is burned completely in excess oxygen, and the mass of the products carefully measured:

Product Mass
Carbon dioxide 24.01g
Water 13.10g

Use this information to find the molecular formula of X.

Answer 1

Molecular formula of X is C₃H₈

Step-by-step explanation:

The compound X reacts producing carbon dioxide, CO₂ and water H₂O.

All carbon of X produce CO₂. Moles of hydrogen are twice moles of water.

Mass of carbon and hydrogen is:

Carbon:

24.01g CO₂ * (1mol / 44g) = 0.5457 moles CO₂ = 0.5457mol C

0.5457mol C * (12g / mol) = 6.5gC

Hydrogen:

13.10g H₂O * (1mol / 18g) = 0.7278moles H₂O * 2 = 1.4556 moles H

1.6375 moles H * (1g/mol) = 1.5g of H

The mass of C + H is 6.5g C + 1.5g H = 8.0g. That means there is no oxygen in the compound X.

Now, empirical formula, the simplest whole number ratio of atoms presents in a molecule (C and H), is:

C = 0.5457mol C / 0.5457mol C = 1

H = 1.4556mol H / 0.5457mol C = 2.667

As empirical formula must be given in whole numbers:

C = 1*3 = 3

H = 2.667*3 = 8

The empirical formula is:

C₃H₈

To confirm that this is the molecular formula of X we need to find the molar mass:

C= 12g/mol*3 = 36g/mol

H= 1g/mol*8 = 8g/mol

Molar mass: 36+8 = 44g/mol

Molecular formula of X is C₃H₈