Formula of Hydrocarbon by Explosion with Oxygen

2010-01-22T00:15:49Z

To 20cm³ of a gaseous hydrocarbon. 80cm³ of oxygen were added. After explosion and cooling to room temperature, the residual gases occupied 70cm³. After absorption by KOH, 30cm³ of oxygen remained. Determine the formula of the hydrocarbon.

Answer by Collin Li for Formula of Hydrocarbon by Explosion with Oxygen

2010-01-22T08:12:41Z

What confused me in this question was:

After absorption by KOH

You don't need to know what this does in VCE Chemistry, but a few Google searches suggested KOH reacts with water and carbon dioxide. This makes sense as a combustion with a hydrocarbon will always produce H₂O and CO₂.

First, let's account for what's gone on:

For the purposes of this question, I will convert cm³ to mL as it's easier to write

Started off with 80 mL of oxygen, and ended up with 30 mL of oxygen (50 mL was used)
We also began with 20 mL of hydrocarbon
Post-explosion, pre-absorption, there was 70 mL of gases (a concoction of all the reactants and products)
The concoction consists of: 30 mL of oxygen, so there was 40 mL of CO₂ and H₂O (we're assuming here that oxygen was in excess by 30 mL, and there's no hydrocarbon left over).

Using the fact that the volume of a gas (V) is directly proportional to the amount of the gas (n) under constant T and P (which is why they said "cooling to room temperature"), then we can say that:

n(hydrocarbon) : n(O₂) = 20 mL : 50 mL = 2 mol : 5 mol
Using 2 mol : 5 mol, then n(CO₂) + n(H₂O) = 4 mol (because there was 40 mL)

Let the hydrocarbon have the formula: C_xH_y.

Combustion reaction: 2C_xH_y + 5O₂ → aCO₂ + bH₂O

We need to find a and b.
We know that: a + b = 4 (from n(CO₂) + n(H₂O) = 4 mol)
To balance the O, we need: 10 = 2a + b

This yields: a = 6, b = -2, which does not make sense.

I'm going to try the question again, with n(CO₂) + n(H₂O) = 7 mol (70 mL of gas, not subtracting the 30 mL of oxygen) now instead, as the question may have forgot to take that into account!

a + b = 7
10 = 2a + b

Result: a = 3, b = 4
Reaction: 2C_xH_y + 5O₂ → 3CO₂ + 4H₂O

Therefore: x = 1.5 and y = 4, which doesn't really make sense, but that means of course the reaction should have been: 2C₃H₈ + 10O₂ → 6CO₂ + 8H₂O

Answer: C₃H₈

Kinda complicated - need to work with two balances simultaneously in order to get this one out, not to mention a sloppily written question that screwed us up the first time! (Note: solving simultaneous equations is unlikely to be required by a VCE Chemistry question - only try this if you love maths!)

Formula of Hydrocarbon by Explosion with Oxygen - Merspi

Formula of Hydrocarbon by Explosion with Oxygen

Answer by Collin Li for Formula of Hydrocarbon by Explosion with Oxygen