[fardeen_gen]

Hello,
When 30 mL of a gaseous hydrocarbon is exploded with 200 mL of oxygen, the volume contraction after explosion was found to be 45 mL. On further treatment with alkaline pyrogallol there was a reduction of 95 mL. Find formula of the hydrocarbon. (All volumes are measured at room temperature).

Can anybody tell me how to approach this problem?(And what is volume contraction?I have never come across this term before.)

[Borek]

Volume contraction means that volume of the gases after reaction is smaller (by given amount) then the amount of reactants.

What gases do you expect to be in the mixture?

You need to calculate total number of moles of gases before and after the reaction first.

What happens when you react gases with alkaline pyrogalol?

[AWK]

The information water was removed completely after combustion is missing. It is assumed all volumes are measured at the same temperature, then volumes and moles are proportional. Assuming a formula of hydrocarbon as C_xH_y write down a balanced reaction of combustion, then calculate a contraction of volume as a function of x and y.

Also write down balanced reactions for alkaline pyrogallol with oxygen and carbon dioxide, then express a reduction of volume after pyrogallol treatment with x and y.

Finally solve a system of two equation with two variables.

[fardeen_gen]

Isn't the volume contraction after explosion = volume of water? Solving, I got y = 3 and x = 33/12.  This is impossible. Still in the deep with this question.

[Borek]

Isn't the volume contraction after explosion = volume of water?

What is pyrogalol for?

Looks to me like there is something fishy about this question.

[Rabn]

Set up the ideal gas equation in initial and final states in terms of volume i.e.:

V_I= nRT/P

V_F= nRT/P

V_F-V_I= 45mL

Then write the chemical equations that correspond to each: i.e.

1) 30mL Hydrocarbon + 200mL Oxygen -> n CO₂ + M H₂O
                      n + m = 185mL

set up the second equation the same way

It should just become a system of equations, something like 4 or 5 unknowns and 4 or 5 equations.  Once you have the correct number of equations you should be able to find moles of hydrocarbon.  It is likely that by using the ideal gas equation and keeping your variables in proper order you can use density to figure out what the hydrocarbon is.

[fardeen_gen]

@Borek: Pyrogallol removes oxygen.
@Rabn: After solving I got y = 3 and x = 33/12. Which is not possible.

[Borek]

@Borek: Pyrogallol removes oxygen.

There is a problem here - solution of pyrogallol is alkalic, so it should remove CO₂ as well, but it looks to me like it has slipped mind of the person asking the question^*.

OK, assuming pyrogallol solution removed oxygen - what was left? What gas and what volume?

--
^* if the gases were adsorbed in alkalic pyrogallol solution, water, excess oxygen and carbon dioxide were removed and the gas that is left can be only excess original hydrocarbon - but its volume is higher then the original volume of hydrocarbon, so obviously something is wrong. Not to mention the fact that you can't have BOTH excess oxygen and excess hydrocarbon at the same time. It will be like legs of Cohen the Barbarian - both too short. Or were it somebody's else legs?

[AWK]

The other error is oxygen used in combustion. Should be air because with alkaline pyrogallol all gas will be absorbed.