A-Level H2 Chemistry
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Tag Archive: Chemistry Questions

Ideal Gas Law: Exam-based Question

Written on August 23, 2010 by Sean in Chemistry Notes & Tips

The following is a question adapted from a JC’s H2 Chemistry Test that was emailed to me by a JC student.

Let’s use the essential concepts that we have learned in the previous blogpost to solve this MCQ question.

Question:

Which gas is most likely to deviate most from ideal gas behaviour?

A. Ar

B. N2

C. CH4

D. HCl

Suggested Answer:

D

Thought Process:

Recalling what we have discussed in previous blogpost, you can will realised that HCl has stronger intermolecular forces of attraction (pd-pd in this case) between its molecules, as compared to weak van der Waals intermolecular forces between N2, Ar and CH4 molecules respectively.

Higher intermolecular forces between molecules –>Deviate most from Ideal Gas Behaviour

Hope you enjoyed yourself here. If you have anything to add, feel free to leave me a comment below.

Cheers! Stay tuned for another question related to this concept.

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Electronic Configurations of Atoms – Part 1 New!

Written on May 15, 2010 by Sean in Chemistry Notes & Tips

We all learned in Basic Chemistry Course/Syllabus such as GCE-O Levels Chemistry that elements are found in the electron shell(s) and each shell can only accomodate up to a maximum of electrons.

This is what you might have learned previously:

  • Shell 1 – Maximum of 2 Electrons
  • Shell 2 – Maximum of 8 Electrons
  • Shell 3 – Maximum of 8 Electrons
  • Shell 8 – Maximum of 8 Electrons etc etc

As such, you would have heard previously that all elements can only hold 2 or 8 electrons in their valence shell – terms such as Duplet and Octet structure exists.

And the Electronic Configurations for Sodium (Ar=11) would be 2.8.1.

Now, when you come to doing Advanced Level Chemistry such as GCE A-Level Chemistry, we are going to learn a little more in depth, and perhaps what you learned in Basic Chemistry Course would not be so true now.

Let’s see what is new and additional!

Electron shells are also known as Principal Quantum Numbers i.e. first shell has principal quantum number = 1

Each electron shell consists of a number of Sub-Shells, labelled s, p, d or f.

The number of sub-shells in each shell equals to the shell number i.e. 1st shell has 1 sub-shell and 2nd shell has 2 sub-shells, etc

Each sub-shell contains a number of Orbitals. in which the electrons are placed. The number of orbitals in each sub-shell depends on the type of sub-shell as shown below:

Type of Sub-Shell Number of Orbitals
s 1
p 3
d 5
f 7

Each of this orbitals can each take up 2 electrons.

Learning something today?

Stay tuned for next blogpost for Part 2 of Electronic Configuration of Atoms!

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Calculation of Ar based on Isotopic Abundance – Challenging Question

Written on May 12, 2010 by Sean in Chemistry Notes & Tips

In the previous blogpost, we have discussed on the Calculation of Ar based on Isotopic Abundance.

Today, we will look at a more challenging question related to it.

I would love to see you attempting the question and leaving your suggested answer in the comments section below.

Question:

Naturally occurring gallium, Ga, is a mixture of two isotopes of mass numbers 69 and 71. What is the percentage abundance of each isotope? Ar of gallium is given as 69.7.

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Calculation of Ar based on Isotopic Abundance

Written on May 8, 2010 by Sean in Chemistry Notes & Tips

In GCE A-Levels, many question on Atomic Structure are based on Isotopes, more specifically it will be testing you on Isotopic Abundance.

We all know that Chlorine has a relative atomic mass, Ar of 35.5. How did we get it?

It is based on the Isotopic Abundance Calculation on its 2 isotopes, namely 35Cl (75% abundance) and 37Cl (25% abundance).

Let’s see how we get the value of 35.5 for its Ar.

Ar =  ∑(Isotopic Mass x % Abudance) / 100

Putting the formulae into action, we will have:

Ar of Cl = [(75 x 35) + (25 x 37)] / 100 = 35.5

Got it?

How about trying out a question on your own?

Question:

Given that Lead has four isotopes: 204Pb, 206Pb, 207Pb and 208Pb. Relative abundance is 2%, 24%, 22% and 52% respectively. Calculate the relative atomic mass of Lead.

PS: Try it out and leave your answer in the comments section below.

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Isotopes & Isotopic Abundance

Written on May 5, 2010 by Sean in Chemistry Notes & Tips

(Photo Credit x-ray delta one)

One of the very important concepts in Chemistry is Isotopes which is defined as “Atoms of the same element with different numbers of neutrons but same number of protons”.

For example, Hydrogen itself has 3 isotopes, each differing in the number of neutrons.

It is importent to know the similarity and differences of Isotopes of An Element. Let’s take a look:

Similarity:

  • Same proton number
  • Same number of electrons in a neutral atom
  • Same electronic configurations
  • Same chemical properties

Differences:

  • Different numbers of neutrons in an atom
  • Different nucleon or mass number
  • Different relative isotopic mass
  • Different physical properties such as melting point, density, etc

Let’s try out a question on Isotopes.

Question:

A sample of hydrogen consists of the isotopes 1H, 2H and 3H. A sample of oxygen only contains 16O.

i) Write the formulae of all the different water molecules that can be formed from these samples. How many different types can you find?

ii) Which molecule has the largest mass?

PS: Try it out and leave the answer in the comments section below. I would love to hear from you.

Stay tuned for the next post – i will share with you more on Calculations involving Isotopic Abundance.

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