Question

. Carbohydrates are the compounds which contain carbon, hydrogen and oxygen. The atomic ratio of H:O is 2:1. When heated in the absence of air, these compounds decompose into carbon and water.
(a) If 310 g if carbohydrate leaves a residue of 124 g of carbon on heating in the absence of air, what is the empirical formula of the carbohydrate?
(b) If 0.0833 mole of the carbohydrate contains 1.0 g of hydrogen, what is the molecular formula of the carbohydrate?

Answer 1

a) Empirical formula : Carbon present

= 124g

water present = 310 - 124 = 186g

Moles of Carbon = 124 / 12 = 10.03

Moles of $H_{2}$O = 186 / 18

= 10.03

Ratio of C : $H_{2}$O = 1 : 1

b) Molecular formula : 0.0833 moles of the carbohydrate contains 1.00g

or one atom of hydrogen. Therefore, one mole of the carbohydrate will contains hydrogne atoms

= 1 / 0.0833

= 12

Number of hydrogen atoms in the molecules being 12, the molecular formula should be six times the empirical formula, i.e.,

C$H_{2}$O × 6

or

$C_{6}H_{12}O_{6}$

Hence, the molecular formula of the carbohydrate is $C_{6}H_{12}O_{6}$