The following experiment was performed in order to determine the formula of a hydrocarbon. The hydrocarbon X is purified by fractional distillation.

0.145 g of X was heated with dry copper (II) oxide and 224 cm³ of carbon dioxide was collected at S.T.P.

(a) Which elements does X contain?

(b) What was the purpose of copper (II) oxide?

(c ) Calculate the empirical formula of X by the following steps:

(i) Calculate the number of moles of carbon dioxide gas.

(ii) Calculate the mass of carbon contained in this quantity of carbon dioxide and thus the mass of carbon in sample X.

(iii) Calculate the mass of hydrogen in sample X.

(iv) Deduce the ratio of atoms of each element in X (empirical formula).

(a) X contains carbon and hydrogen as it is a hydrocarbon.

(b) The purpose of copper (II) oxide is to act as an oxidizing agent.

(c) (i) 22400 cm³ CO₂ has mass = 44 g

∴ 224 cm³ CO₂ will have mass = $\dfrac{44}{22400}$ x 224 = 0.44 g

Molar mass of CO₂ = 12 + 2(16) = 12 + 32 = 44 g

Moles of CO₂ = $\dfrac{\text{Mass}}{\text{{Molecular mass}}}$ = $\dfrac{0.44}{44}$ = 0.01 moles

(ii) Mass of carbon in 44 g CO₂ = 12 g

∴ Mass of carbon in 0.44 g CO₂ = $\dfrac{12}{44}$ x 0.44 = 0.12 g

As X contains carbon and hydrogen, so sample X has 0.12 g of carbon

(iii) Mass of Hydrogen in X = 0.145 - 0.12 = 0.025 g

(iv) Ratio of moles of C : H

= $\dfrac{0.01}{0.01}$ : $\dfrac{0.025}{0.01}$

= 1 : $\dfrac{25}{10}$

= 1 : $\dfrac{5}{2}$

= 2 : 5

Hence, ratio of C : H = 2 : 5

so, the empirical formula of hydrocarbon is C₂H₅