# As level cambridge question. Plz help!!

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Thread starter 3 years ago
#1
The complete combustion of 2 moles of a straight chain alkane produces 400 dm3 of carbon dioxide measured at 301K and 1*10^5 . Carbon dioxide can be assumed to behave as an ideal gas under these conditions.

What is the formula of the straight chain alkane
1
3 years ago
#2
The conditions you have listed are room temperature conditions, for which 1 mol of any gas has a volume of 24dm3. 400/24 = mol of carbon dioxide produced then you know how many carbons there are and since its straight chain the general formula for an alkane applies
0
3 years ago
#3
(Original post by BDunlop)
The conditions you have listed are room temperature conditions, for which 1 mol of any gas has a volume of 24dm3. 400/24 = mol of carbon dioxide produced then you know how many carbons there are and since its straight chain the general formula for an alkane applies
I dont think you can assume that 1 mole of gas has a volume of 24dm3 because its not standard conditions (293K,100kPa). But if u use the formula NRT=PV u should get a value for the number of moles of co2
0
3 years ago
#4
(Original post by BDunlop)
The conditions you have listed are room temperature conditions, for which 1 mol of any gas has a volume of 24dm3. 400/24 = mol of carbon dioxide produced then you know how many carbons there are and since its straight chain the general formula for an alkane applies
Its not quite RTP (by a few degrees), indeed the ideal gas equation gives a different answer.

I would suggest using n = PV/RT
0
3 years ago
#5
(Original post by charco)
Its not quite RTP (by a few degrees), indeed the ideal gas equation gives a different answer.

I would suggest using n = PV/RT
Isn't room temperature given as 28°C

Apologies, just looked up, you're right
0
Thread starter 3 years ago
#6
(Original post by charco)
Its not quite RTP (by a few degrees), indeed the ideal gas equation gives a different answer.

I would suggest using n = PV/RT
So how to get the final answer ?
0
3 years ago
#7
so firstly, i used the formula n=PV/RT
convert all units to correct ones, so dm3>m3 is /1000
400/1000 = 0.4

using the formula - 0.4*(1*10^5)/301*8.31 ( it said ideal gas condition) = X

since the initial stated it required 2 moles of the alkane to produce this c02, therefore, /2 to calculate the singular molar for it, which results in 8 carbons (7.99)
from the options available, it would be c8h18.
you could also substitute a reaction to confirm such as,
2 c8h18 + 25 o2 -> 16 c02 + 18h20
2
1 year ago
#8
C6h12
0
X

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