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## Question

secondary 3 | A Maths

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Why

The options indicate that the gas contains carbon and hydrogen atoms only.

In other words, it is a hydrocarbon.

Let the chemical formula of the hydrocarbon be CxHy.

Write the chemical equation, keeping in view that CxHy ≡ 5 O2

CxHy + 5O2 → y/2 H2O + xCO2

The equation must be balanced. Number of oxygen atoms on both sides must be the same.

5 × 2 = y/2 × 1 + x × 2

10 = ½y + 2x

20 = y + 4x

y = 20 - 4x

Now the options indicate that the gas is an alkane. Remember that alkanes have general formula CnH(2n+2)

So y = 2x + 2

Solving for x,

20 - 4x = 2x + 2

6x = 18

x = 3

y = 2(3) + 2 = 8

C3H8 (propane) is the answer.

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