Empirical And Molecular Formulae

Question 1

Define Empirical formula of a compound

Answer 1

This is the simplest formula which expresses the ratio of the number of atoms of elements present in a compound

Question 2

What is molecular formula of a compound?

Answer 2

This is the formula showing the exact number of atoms of every element present in one molecule of the compound.

Question 3

Describe the process of determination of the molecular formula of hydrocarbons by sparking in excess oxygen (Eudiometry)

Answer 3

Procedure
- A known volume (V cm3) of gaseous hydrocarbon is mixed with excess oxygen (V1cm3).

• The gaseous mixture is then exploded by sparking forming carbon dioxide and steam.
• After complete combustion of the hydrocarbon, the gaseous mixture is cooled to the original temperature and pressure and the steam condensed into water whose volume is negligible.
• The volume (V2 cm3) of the remaining gaseous mixture consisting of unreacted oxygen and liberated carbon dioxide measured and recorded.
• The carbon dioxide gas in the mixture absorbed by adding a little concentrated sodium hydroxide solution or concentrated potassium hydroxide leaving behind unreacted oxygen.
• The final volume consisting of unreacted oxygen (V3 cm3) is measured and recorded.

Calculations
- Equation for the reaction: 𝐶𝑥𝐻𝑦(𝑔) + (𝑥 + 𝑦4) 𝑂2(𝑔) → 𝑥𝐶𝑂2(𝑔) + 𝑦2 𝐻2𝑂(𝑙)
𝒗 𝑐𝑚3 𝒗 (𝒙 + 𝒚𝟒) 𝑐𝑚3 𝒗𝒙 𝑐𝑚3 𝒏𝒆𝒈𝒍𝒊𝒈𝒊𝒃𝒍𝒆

• 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑐𝑎𝑟𝑏𝑜𝑛 𝑑𝑖𝑜𝑥𝑖𝑑𝑒 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 = 𝑉𝑥 𝑐𝑚3 = (𝑉2 − 𝑉3) 𝑐𝑚3 𝑎𝑛𝑑 𝒙 = (𝑽𝟐 − 𝑽𝟑) 𝑽
• 𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛 𝑐𝑜𝑛𝑠𝑢𝑚𝑒𝑑 𝑑𝑢𝑟𝑖𝑛𝑔 𝑐𝑜𝑚𝑏𝑢𝑠𝑡𝑖𝑜𝑛 = 𝑉 (𝑥 + 𝑦4) 𝑐𝑚3 = (𝑉1 − 𝑉3) 𝑐𝑚3 𝑎𝑛𝑑 𝒚 = 4 (𝑉1 − 𝑉3 − 𝑥)𝑉
• Substituting into the general formula, CxHy, the molecular mass of the hydrocarbon is obtained.