Question

The following experiment was performed in order t odeterminethe formula of a hydrocarbon. The hydrocarbon X is purified by franctional distillation
0.145 g of X were heated with dry copper (II) oxide and 224 $c{m}^3$ of carbon dioxide was collected at STP.
(a) Which element 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) Reduce the ratio of atoms of each element in X (empirical formula).

Answer 1

a) The hydrocarbon has only Carbon and Hydrogen

b) Here, CuO is an oxidizing agent

c). Find the empirical formula of X by the following steps:

i).We have given the volume of $C{O}_2$ gas = 224 cm3 = 224ml = 0.224 L

We know at STP, 22400 ml = 1 mole

The moles of $C{O}_2$ gas = $\frac{0.224L}{22.4L/mol}=0.01molesofCO2gas$

ii). One moles of $C{O}_2$ has 1 mol C. that means Moles of C = Moles of $C{O}_2$.

Mass of the Carbon = Moles x Atomic mass of C

= 0.01 moles C x 12 g/mol = 0.12 grams C

iii). mass of Hydrogen = Mass of Hydrocarbon - mass of carbon = 0.145 - 0.12 = 0.025 grams H

iv). Moles of Hydrogen = 0.025 g/1g/mol = 0.025 moles H.

The ratio of the moles: C = $\frac{0.01}{0.01}=1$

The ratio of the moles: H = $\frac{0.025}{0.01}=2.5$

So the ratio of C : H is 1 : 2.5 or 2 : 5

The empirical formula X is: $C_2{H}_5$