## 2002 - Multiple Choice - Q11

### jamiep

Posted 30 May 2005 - 11:12 AM

No clue about 2002 Q11:

The equation for the complete combustion is C3H8(g)+5O2(g) -> 3CO2(g) + 4H20(l)

50cm3 of propane is mixed with 500cm3 of oxygen and the mixture is ignited. What is the volume of the resulting gas mixtue?

A 150cm3 B 300cm3 C 400cm3 D 700cm3

I thought 1mole reacted with 1 mole (oxygen is in excess) so 50cm3 propane -> 150 cm3 of Carbon dioxide so the answer would be A 150cm3. I cant understand why the answer is C 400 cm3.

PS Sorry about all these last minute questions, getting quite nervous now!!

### bred

Posted 30 May 2005 - 11:19 AM

Remember there is also unreacted O2.

### tupacshakur

Posted 30 May 2005 - 11:21 AM

i think the answer is 400cm

### jamiep

Posted 30 May 2005 - 11:28 AM

Could you explain why the answer is 400cm3?

### bred

Posted 30 May 2005 - 11:31 AM

50cm3 propane and 250cm3 oxygen (5x50) reacts, and 100cm3 of oxygen is left unreacted (5mol-3mol=2mol, 2x50=100).

### jamiep

Posted 30 May 2005 - 11:35 AM

QUOTE(tupacshakur @ May 30 2005, 11:21 AM)
i think the answer is 400cm

QUOTE(bred @ May 30 2005, 11:31 AM)
50cm3 propane and 250cm3 oxygen (5x50) reacts, and 100cm3 of oxygen is left unreacted (5mol-3mol=2mol, 2x50=100).

I did not understand that it was asking for me to add on the oxygen. Hope I don't get tricked tomorrow!

### dfx

Posted 30 May 2005 - 02:42 PM

Normally you just ignore the reactant in excess and use the other reactant to figure out the calculations, but the question asks for the TOTAL VOLUME, so I suppose you do have to take into account the unreacted oxygen.