Degree of Unsaturation: Worked Solution - Organic Chemistry Quiz 4

This post is the worked solution for organic chemistry Quiz 4 which talks about the degree of unsaturation of organic compounds. If you have not tried this question, you may like to visit this post first.

Saturated vs Unsaturated Compounds

From the alkanes and alkenes chapter, we learn that alkanes have only single bonds and are saturated compounds. On the other hand, alkenes have double bonds, and are unsaturated compounds. A common test for saturation (or to distinguish alkanes from alkenes) is to add aqueous bromine (in the dark). Alkenes will turn reddish brown bromine colorless. For alkanes, bromine will remain reddish brown.

This video from youtube is a good illustration of the test:



Formula of Alkane and Alkene

In O Levels, they generally tell you that alkane has a general molecular formula of CnH2n+2, while alkene has a general molecular formula of CnH2n. Note that the alkene general molecular formula of CnH2n only applies to straight chain alkene with just one double bond. What happens when there is more than one double bond?

Every additional double bond you have will decrease the number of hydrogen atoms by 2.

Worked Solutions to Quiz 4

For a straight chain alkane molecule with 20 carbon atoms, we would expect 42 hydrogen atoms (using the formula of CnH2n+2. For every double bond you have, the number of hydrogen atoms will decrease by 2. Since there are 30 hydrogen atoms in this hydrocarbon, we will expect 6 double bonds.

Number of double bonds = (42 - 30)/2 = 6

One mole of double bond will react with one mole of bromine. With 6 double bonds, it means 6 moles of bromine.



Organic Chemistry - Quiz 4

This quiz for this week is on organic chemistry. You may like to revise the chapter on alkanes and alkenes, as well as the chapter on alcohols, carboxylic acids, esters and macromolecules first before attempting the question. 





A straight chain hydrocarbon has a molecular formula of C20H30. How many moles of bromine will react with one mole of this compound?

a. 4
b. 5
c. 6
d. 7



Hint: This question is all about looking at the degree of unsaturation in organic compounds. Why is the general formula for alkane CnH2n+2, while that for an alkene (with a single double bond) is CnH2n?

Full worked solution for this quiz can be found here.

Exams are round the corner

The end of the year signifies major examinations before the holidays. Are you ready for it?

October till mid November is the period where the GCE Chemistry (and of course other subjects too), which is just on an average, a month from now.

Are you ready?


After Secondary School Chemistry

I was talking to my student who had just graduated from secondary school, and is moving on to junior college (and taking A levels next year). Her comments was that chemistry for A levels is almost foreign to her. Many topics are almost totally new. Indeed, A level chemistry is much more in depth than secondary school. And with  that, many students may feel lost as the topics seem so different.

I was considering setting up a new blog on A Level Chemistry, or to extend the current blog to include JC topics. What do you prefer?

choose


Leave a comment below or send me a message to let me know.

Cheers!

The Learning Experiment - Organizing and Memorizing (Part 3) - Using Mole Concept as Example

In this post, I will be using the chapter on mole concept as an example of how to organize information for easy memorization.

If you take a closer look back at the post on mole concept formulae, you would notice that I have arranged the formulae according to the states (i.e. solutions, gases, for all states). This is an example of how you can organize information for easy memorization. 

As such, you should always remember that this formula is always application for all situations, whether solids, liquids, gases, solutions:
  • mole = mass/ (molar mass)
  • mole = (number of particles) / (Avogadro's constant)


When you see solutions (aqueous state), the following applies:
  • mole = concentration in mol/dm3 x volume in dm3
  • mass = concentration in g/dm3 x volume in dm3

When you see gases, the following apply:
  • mole = volume of gas / molar volume 

Now, you have three groups of information, and each group has 1 to 2 formula(e), making memorization and recalling them easier.

Related Post:
- The Learning Experiment - Organization and Memorization (Part 1)
The Learning Experiment - Organization and Memorization (Part 2)
- Mole Concept - Learning from Example
- Mole Concept Formulae

Related Questions:
- Quizzes and Tests on  Mole Concept