💯An Ingelious Way to Solve Mole Concept Combustion Questions

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Mole Concept Combustion Questions

Here’s an “ingelious” way to solve combustion questions related to mole concept using model drawings (yep, like in primary school). Let’s look at a typical JC A Level H2 Chemistry question:

When 20 cm3 of a gaseous hydrocarbon were sparked with 150 cm3 of oxygen and the residual gases cooled to room temperature, a contraction of 60 cm3 occurred. A further contraction of 80cm3 took place when the residual gases were subjected to aqueous NaOH. Find formula of the hydrocarbon. [All gaseous volumes are measured under identical conditions.]

Proposed solution:

Mole concept combustion
Use model drawings to visual the problem easily!

CxHy + (x + \frac{y}{4}) O2 \rightarrow CO2 + H2O

V(CO2) produced = 80 cm3 since it is removed by alkaline NaOH.

\frac{V(CO_2)}{V(C_xH_y)} = \frac{80}{20} = \frac{x}{1} \Rightarrow x = 4

Total initial volume before combustion = 150 + 20 = 170 cm3

Since a contraction of 60cm3 occurred after ignition and cooling, contraction = total initial volume – total final volume

V(O2) excess + V(CO2) = 170 – 60 = 110 cm3.

V(O2) excess = 110 – 80 = 30 cm3

V(O2) reacted = 150 – 30 = 120 cm3

\frac{V(O_2)_{reacted}}{V(C_xH_y)} = \frac{120}{20} = \frac{x+\frac{y}{4}}{1} = \frac{4+\frac{y}{4}}{1} \Rightarrow  y = 8

The molecular formula is C4H8.

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