Can anyone do this question? Thanks

Watch this thread
Azoy
Badges: 4
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#1
Report Thread starter 11 months ago
#1
Complete combustion of 1 mol of hydrocarbon X requires exactly 8.5 mol of oxygen.
Incomplete combustion of 1 mol of hydrocarbon X, to form carbon monoxide and water requires exactly 5.5 moles of oxygen.
How many hydrogen atoms are in one molecule of hydrocarbon X?
A)6
B)8
C)10
D)12
E)12

The answer is C) 10 but no idea why
Thanks
0
reply
RavenclawAlto
Badges: 5
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#2
Report 11 months ago
#2
Say X has A carbons and B hydrogens. Complete combustion makes A carbon dioxide, requiring 2A oxygen atoms, and 1/2B waters requiring 1/2B oxygen atoms. There are two oxygen atoms in 1 oxygen molecule, therefore 2×8.5=17=2A 1/2B. Incomplete combustion forming carbon monoxide makes A carbon monoxide requiring A oxygen atoms and 1/2B waters requiring 1/2B oxygen atoms. Therefore 2×5.5=11=A 1/2B. Solving these equations simultaneously 17-11=2A-A 1/2B-1/2B which gives 6=A. Then substitute A=6 into 11=A 1/2B gives 1/2B=5 so B=10 and B was the number of hydrogen atoms in X so that's your answer. I know that was very mathematical, but you could follow a similar process without expressing it so mathematically, or possibly solve it by looking at and balancing the equations of complete and incomplete combustion, but this was the easiest way to type it out. If you still don't get it, let me know what you don't understand.
2
reply
deskochan
Badges: 11
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#3
Report 11 months ago
#3
CxHy + (x+y/4) O2 --> xCO2 + y/2 H2O (complete combustion)
CxHy + (x/2+y/4) O2 --> xCO +y/2 H2O (incomplete combustion)
x+y/4 =8.5
x/2+ y/4 =5.5
solving equation, x = 6, y = 10, thus H = 10
2
reply
Azoy
Badges: 4
Rep:
? You'll earn badges for being active around the site. Rep gems come when your posts are rated by other community members.
#4
Report Thread starter 11 months ago
#4
(Original post by RavenclawAlto)
Say X has A carbons and B hydrogens. Complete combustion makes A carbon dioxide, requiring 2A oxygen atoms, and 1/2B waters requiring 1/2B oxygen atoms. There are two oxygen atoms in 1 oxygen molecule, therefore 2×8.5=17=2A 1/2B. Incomplete combustion forming carbon monoxide makes A carbon monoxide requiring A oxygen atoms and 1/2B waters requiring 1/2B oxygen atoms. Therefore 2×5.5=11=A 1/2B. Solving these equations simultaneously 17-11=2A-A 1/2B-1/2B which gives 6=A. Then substitute A=6 into 11=A 1/2B gives 1/2B=5 so B=10 and B was the number of hydrogen atoms in X so that's your answer. I know that was very mathematical, but you could follow a similar process without expressing it so mathematically, or possibly solve it by looking at and balancing the equations of complete and incomplete combustion, but this was the easiest way to type it out. If you still don't get it, let me know what you don't understand.
Thanks!
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest

Were exams easier or harder than you expected?

Easier (36)
26.47%
As I expected (45)
33.09%
Harder (48)
35.29%
Something else (tell us in the thread) (7)
5.15%

Watched Threads

View All