Determination of Molecular Formula of Hydrocarbons using Combustion Data

Earlier, we have discussed on the strategy of Determining the Molecular Formula of Compounds using Composition by Mass.

Today, we will discuss on the Determination of Molecular Formula of Hydrocarbons using Combustion Data. This is a new concept for those that making their transition from GCE O-Levels to GCE A-Levels and thus will be one of the key questions to be asked in GCE A-Level H1 and H2 Chemistry Examinations.

Let’s look at the method involved.

Hydrocarbons burns in excess oxygen based on the following equation:

CxHy (g) + (x + y/4) O2 (g) –> xCO2 (g) + y/2 H2O (l)

At room temperature conditions or s.t.pm the water product is actually a liquid. Thus, the volume is negligible compared to volumes of CxHy (g), O2 (g) and CO2 (g).

Using Volume Ratio;

Hence, if 1cm3 of  is completely burned in oxygen,

Volume of oyxgen used = (x + y/4) cm3

Volume of carbon dioxide produced = x cm3

Volume of water produced (as liquid) = 0 cm3

Let’s check out an exam-based question to see how we can make use of the above to solve question.

Example:

10cm3 of a gaseous hydrocarbon required 20 cm3 of oxygen for complete combustion. 10 cm3 of carbon dioxide was produced. Calculate the molecular formula of the hydrocarbon.

and

Suggested Solution:

Using the general equation and applying volume ratio, we have

CxHy (g) + (x + y/4) O2 (g) –> xCO2 (g) + y/2 H2O (l)

10cm3…… 20cm3 ……………….10cm3

1mol             2 mol                              1 mol

Based on the above comparision, we have

x = 1 and (x + y/4) = 2

Solving it gives y = 4

Hence, the molecular formula of the hydrocarbon is CH4.

Hope the above explanation is useful to you. Do note that this is a very important concept when you are in JC1 and will still be tested when you are in JC2. I realised that many of my students in my GCE A-Level H2 Chemistry Tuition Classes are not well trained in their Junior Colleges when it comes to this basic concept.

PS: Let me know how you find it. Drop me a comment. I would love to hear from you. :-D

Many people talked about teaching Chemistry, I simply LOVE it. I am a passionate Chemistry Coach based in Singapore, Southeast-Asia and aspire to be one of the most dynamic, powerful and humorous speakers in Asia. My 16+ years of coaching experience has equipped me to understand the true reasons why students are not able to perform well in Chemistry, and allow me to structure my teaching methodology to cater to different levels of learners. If you have found this post useful, please share it with your friends. I would really appreciate it!

12 Responses to Determination of Molecular Formula of Hydrocarbons using Combustion Data
  1. zain
    October 2, 2010 | 6:02 pm

    hey. that was really helpful.

  2. Pennyproud
    December 12, 2010 | 7:14 pm

    It was helpful but would need to know how to cal.. more complex ones..

  3. steve
    January 23, 2011 | 7:03 am

    thanks could you please put up more complex examples?

  4. ben
    June 1, 2011 | 5:53 pm

    thanx, very useful

  5. deshamuka
    June 14, 2011 | 4:27 am

    what if you consider the water evolve as water vapour and it is a considerable amount??

  6. lorluh
    September 30, 2011 | 2:06 pm

    i have been searching 4 a solution for a very long time,thanks that was really helpful!

  7. UJJAINI.R
    October 5, 2011 | 2:24 pm

    CUD U PLZ PUT UP SOME MORE DIFFICULT EXPLS LIEK TEH ONE AM STUCK WITH IT GOES LIKE: A HYDROCARBON OF 20.CM^3 AND 100C^3 OF OXYGEN IS EXPLODED. WHEN COOLED THE VOL OF TEH GAS MIX WAS LEFT TO 70.0CM^3 . WHEN PASSING THROUGH aq NAOH THE VOL WAS REDUCED BY 60CM^3. DETERMINE THE EMPIRICAL FORMULA OF THE HYDROCARBON.
    ITLL BE HELPFUL IF U CAN SEND AN EX OF THIS KIND!
    HOPE IAM NT OFFENDING U IN ANYWAY
    THANKU

  8. NAYEM
    June 25, 2012 | 2:45 am

    IT IS VERY EASY MATTER……..BUT WE NEAD MORE EXAMPLE TO PRACTISE..

  9. Diksha
    June 20, 2013 | 10:06 pm

    Thank u very much.. This helped me solve a question I stacked at..

    Thank u..:-)

  10. Theophile
    August 2, 2013 | 7:52 am

    thanks a lot

  11. Ammy
    August 23, 2013 | 7:00 pm

    Hmmmmm! I actually enjoyed the way u simplified it thanks, but pls could u give more examples on some difficult one

  12. Sean
    October 25, 2013 | 3:56 pm

    Thanks everyone for the encouragement and support.

    I’m glad you like my simple way of presentation.

    Yup, i always do my best to make abstract complicated A-Level Chemistry concepts to be as simple as possible, so that my students in my JC A Level H2 Chemistry classes can benefit from it.

    That’s how i will continue to write on this blog also.

    Hope it can help other students that are studying overseas as well as other Singapore JC students who are not able to attend our tuition classes.

    Keep sharing, Keep learning!

    Sean Chua :)

Leave a Reply

Wanting to leave an <em>phasis on your comment?

Trackback URL http://www.ALevelH2Chemistry.com/determination-of-molecular-formula-of-hydrocarbons-using-combustion-data/trackback/