chemistry SOS!

• mystiv

  3 Oct 05, 22:10

  >.< need help guys. cant figure out why
  
  Which of the following is the molecular formula of a hydrocarbon which contains 85.7% carbon and 14.3% hydrogen by mass?
  
  a) CH2
  b) CH4
  c) C2H2
  d) C2H4
  e) C2H6
  
  answer is D, but why is it D and not A? >.<
• sinicker

  3 Oct 05, 22:22

  Originally posted by mystiv:

  >.< need help guys. cant figure out why
  
  Which of the following is the molecular formula of a hydrocarbon which contains 85.7% carbon and 14.3% hydrogen by mass?
  
  a) CH2
  b) CH4
  c) C2H2
  d) C2H4
  e) C2H6
  
  answer is D, but why is it D and not A? >.<

  this is a trick question.
  remember, C requires 4 covalent bonds to be stable. thus, Ch2 does not exist. hence, answer is ethene.
• SBS9818A

  3 Oct 05, 22:23

  Were you given the molecular mass? It should give you some form of clue.
  
  If not, the ans is still D since CH2 doesnt exist. CH2 and C2H4 have same % ratio of C and H atoms.
• mystiv

  3 Oct 05, 22:26

  thanks!
• alexkusu

  3 Oct 05, 22:31

  CH2 x 2 =C2H4
  
  btw, CH2 is an impossibility
• kaobeikaobu

  3 Oct 05, 22:33

  is this sec 2 qn?
• mystiv

  3 Oct 05, 22:39

  Originally posted by alexkusu:

  CH2 x 2 =C2H4
  
  btw, CH2 is an impossibility

  o.O why x2?
  
  and yeah, i realized the CH2 part
  
  sec3
• miaomiao11

  3 Oct 05, 22:40

  find the no. of moles then c which one has the same ratio lo... simple ma..
• alexkusu

  3 Oct 05, 22:48

  Originally posted by mystiv:

  o.O why x2?
  
  and yeah, i realized the CH2 part
  
  sec3

  ratio thing.. since u choose A ..but its impossible..so times it up to get C2H4 which is possible. hence the answer
  
• miaomiao11

  3 Oct 05, 23:28

  Originally posted by alexkusu:

  ratio thing.. since u choose A ..but its impossible..so times it up to get C2H4 which is possible. hence the answer
  

  +1
• the.raven

  4 Oct 05, 07:11

  CH2?
  
  methene?
• Fatum

  4 Oct 05, 08:14

  Originally posted by sinicker:

  this is a trick question.
  remember, C requires 4 covalent bonds to be stable. thus, Ch2 does not exist. hence, answer is ethene.

  It should be Ethane ... not Ethene ....
  
  Alkanes follow the general rule C(x)H(2X + 2) .....
  
  Alkenes follow the general rule C(x)H(2x) ....