# Can anyone do this question? Thanks

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#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
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
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
#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
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