Question

1. a gaseous hydrocarbon was found to decolourise an acidified potassium manganate (vii) solution. when two moles of q are burnt completely in air, six moles of carbon (iv) oxide and six moles of water were formed.

a) write the structural formula of q (1 mark)

b) name the homologous series to which q belongs (1 mark)

c) name one industrial source of q (1 mark)

Explanation:

🆕 New Concept Discovered: Hydrocarbon Combustion & Identification
Determining molecular formulas from combustion products.

Step 1: Determine the molecular formula of Q

We are given that when \(2\) moles of the gaseous hydrocarbon \(Q\) are burnt completely in air, \(6\) moles of carbon(IV) oxide (\(\text{CO}_2\)) and \(6\) moles of water (\(\text{H}_2\text{O}\)) are formed.

Let the molecular formula of the hydrocarbon be \(\text{C}_x\text{H}_y\). The combustion equation for \(2\) moles of \(Q\) is:

$$ 2\text{C}_x\text{H}_y + \text{O}_2 ightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} $$

By balancing the atoms on both sides:

• Carbon atoms:

$$ 2x = 6 \implies x = 3 $$

• Hydrogen atoms:

$$ 2y = 6 \times 2 = 12 \implies y = 6 $$

Thus, the molecular formula of \(Q\) is \(\text{C}_3\text{H}_6\).

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Step 2: Write the structural formula of Q

The molecular formula \(\text{C}_3\text{H}_6\) corresponds to propene (an alkene) or cyclopropane. Since the problem states that \(Q\) is a gaseous hydrocarbon that decolourises acidified potassium manganate(VII) solution, it must contain a carbon-carbon double bond (\(\text{C}=\text{C}\)). This confirms \(Q\) is propene.

The structural formula of propene (\(\text{CH}_3\text{CH}=\text{CH}_2\)) is:

    H   H   H
    |   |   |
H - C - C = C - H
    |
    H

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Step 3: Identify the homologous series of Q

Since \(Q\) has the general formula \(\text{C}_n\text{H}_{2n}\) and contains a carbon-carbon double bond, it belongs to the alkenes homologous series.

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Step 4: Identify an industrial source of Q

Propene is produced industrially primarily through:

• Cracking of crude oil fractions (specifically, thermal cracking or catalytic cracking of petroleum).
• Natural gas processing.

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Answer:

a) Structural formula of Q:

    H   H   H
    |   |   |
H - C - C = C - H
    |
    H

(or written condensed as \(\text{CH}_3\text{CH}=\text{CH}_2\))

b) Homologous series:

Alkenes

c) Industrial source:

Cracking of petroleum (crude oil)