The Student Room Group

Chemistry question

I don’t understand this question

Which process has the highest atom economy for preparing ethene, C2H4?
In each process, assume that ethene is the only product that is used.
A C10H22
B C6H14
C C2H5Cl
D C2H5OH
Your answer


And this question

Complete combustion of 0.050mol of an alkane produces 5.40g of H2O. What is the molecular formula of the alkane?
A C2H6
B C3H8
C C4H10
D C5H12
Reply 1
Original post by ilayda 1234
I don’t understand this question

Which process has the highest atom economy for preparing ethene, C2H4?
In each process, assume that ethene is the only product that is used.
A C10H22
B C6H14
C C2H5Cl
D C2H5OH
Your answer


And this question

Complete combustion of 0.050mol of an alkane produces 5.40g of H2O. What is the molecular formula of the alkane?
A C2H6
B C3H8
C C4H10
D C5H12

i’m not sure about the first one, i think the answer would be A tho if the question is about fracking

for the second one, the answer should be pentane and here is the explanation:

the general formula for an alkane is CnH2n+2
from this, you can make a general equation for the complete combustion of an alkane which would look like this
2(CnH2n+2)+ 3n+1(O2) ==> 2n+2(H2O) + 2n(CO2)

the numbers outside the brackets are the molar ratios btw
so basically from the question u can get the moles of water which would be mass/molar mass so that’s 5.4/18 = 0.3 mol of water produced

we know that there was originally 0.05 mol of alkane in the system. from the equation we made above, that basically means the molar ratios of the alkane to the water are equivalent to 0.05:0.3
this gives the following equation:
0.05/0.3=2/(2n+2)
rearrange to get
0.05*(n+1)=0.3
and then
n+1=0.3/0.05=6
so n=5

if you remember, we said at the start n is the number of carbons so we have a pentane !!
hope that helped lols
Original post by ilayda 1234
I don’t understand this question

Which process has the highest atom economy for preparing ethene, C2H4?
In each process, assume that ethene is the only product that is used.
A C10H22
B C6H14
C C2H5Cl
D C2H5OH
Your answer


And this question

Complete combustion of 0.050mol of an alkane produces 5.40g of H2O. What is the molecular formula of the alkane?
A C2H6
B C3H8
C C4H10
D C5H12


First and foremost, do you know what the equation used to calculate atom economy is?

Next, consider what reactions are being conducted in order to actually form ethene from each of the above compounds and perhaps try writing equations for C and D as they will be useful.

It will also be useful to identify the type of reaction taking place when either A or B is used to make ethene. Why do you think I have not suggested trying to write equations for these particular reactions?
(edited 4 months ago)

Quick Reply

Latest