Question

12 mL of a gaseous hydrocarbon was exploded with 50 mL of oxygen. The volume
measured after explosion was 32 mL. After treatment with KOH the volume
diminished to 8 mL. Determine the formula of the hydrocarbon​

Answer 1

Answer:

The molecular formula of hydrocarbon is $C_{2} H_{8}$

Explanation:

The combustion reaction is as follows:

$C_{x} H_{y}+O_{2} \rightarrow C O_{2}+H_{2} O$

Volume of $\left(CO_{2}+\text {unreacted } O_{2}\right)=32 \mathrm{ml}$

$C O_{2}$ is absorbed by KOH , the given volume of KOH is 8ml

Final volume of $C O_{2}=32-8=24$

The ratio of volume of gases are 12: 50: 28

                                                           6: 25: 12

                                                            1: 4.1: 2

• All the carbon from the hydrocarbon is in the $C O_{2}$, from the ratio one mole of hydrocarbon produces two moles of carbon.

• All the H end up in the water. One moles of hydrocarbon produce four moles of hydrogen atoms. But also each water molecule has ‘two atoms of hydrogen’.
• So, one mole of hydrocarbon has ‘eight moles of hydrogen’.

Therefore “molecular formula of hydrocarbon” is $C_{2} H_{8}$

Answer 2

Explanation:

CxHy + O2 →CO2 + H2O

Volume of (CO2 + unreacted O2) = 32 ml.

As CO2 can be absorbed by KOH . When this 32 ml. mixture is treated with KOH, it is reduced to 8 ml.

Therefore amount of CO2 formed = 32 ml. - 8ml.= 24 ml.

Amount of unreacted O2 = 8 ml.

Volume of O2 reacted = 50 ml - 8 ml = 42 ml.

Volume of hydrocarbon = 12 ml.

Apply Principle of Atom Conservation (POAC) on C we get:

Xcross times moles of Cx Hy = 1cross times no. of moles of CO2

X ×12 ml = 1 x 24 ml

X = 24/12 = 2

Apply POAC on H atoms

Y × no. of moles of Cx Hy = 2× no. of mole of H2O -----------------( 1 )

Apply POAC on O atoms

2 cross times no. of moles of O2 = 1× no. of mole of H2O + 2 × no. of mole of CO2

From eq. (1)

2 cross times no. of moles of O2 = y/2× no. of mole of Cx Hy + 2 × no. of mole of CO2

2×42 = Y/2×12 + 2 *24

84 = 6Y + 48

84-48 = 6Y

36 = 6Y

Y = 6

So the formula of hydrocarbon = C2H6