Question

A mixture of C3H8(g) & O 2
having total volume 100 ml in an Eudiometry tube is sparked & it is observed that a contraction of 45 ml is observed what can be the composition of reacting mixture?
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Answer 1

Answer:

The balanced equation is: C3H8 + 5O2 → 3CO2 + 4H2O. Thus 6 moles goes to 7 moles of which only 3 moles remain uncondensed in the gaseous state while the water condenses to an insignificant volume. Thus for a stoichiometric ratio we would see the volume of 100 ml decrease by 50 ml, as 6 moles of gas goes to 3 moles of gas. But we are told the volume decreases by 45 ml. We can say therefore that the stoichiometric volume of propane and oxygen would be 45/0.5 = 90 ml. Therefore we have an extra 10 ml of gas to make the 100 ml in the question. This extra 10 ml of gas can be oxygen or propane, as either way the excess does not get used and is a “spectator” to the reaction. The composition of the mixture is either (i) 15 ml propane and 85 ml oxygen or (ii) 25 ml propane and 75 ml oxygen.

Now some caveats.

(i) The eudiometer tube needs to be made of thick metal because a glass one will explode sending glass shards everywhere. A quick calculation suggests a pressure in excess of 10 bar plus a shock wave pressure that I cannot guess at. You cannot read the volume inside a metal tube so some clever design will be needed.

(ii) The formed carbon dioxide will partially dissolve in the produced water when it comes down from flame temperature of > 2000 deg C odd thus reducing the volume by more than the 45 ml in the question.

A little note of caution. The real world is often a lot different from the idealised world and those who exclusively inhabit the latter can get into serious trouble.