Most Junior Colleges and Centralised Institute in Singapore will start their GCE A-Level H2 Chemistry curriculum (syllabus code: 9729) with Physical Chemistry, and the most common topic will be Atoms, Molecules and Stoichiometry.
(Photo courtesy Rocker Dude)
This is essentially a re-cap of what the students have learned during their GCE O-Level Chemistry studies and they include sub-topics such as:
- Relative masses of Atoms & Molecules
- Definition of Mole in terms of Avogadro Constant
- Relative Atomic Mass based on Relative Abundances of its Isotopes
- Calculation of Empirical & Molecular Formulae
- Calculations using Mole Concepts involving Reacting Masses, Volume of Gases and Concentrations of Solutions
However, the questions given in their tutorials and tests are usually pitched at a more challenging level and i realised that many Junior College (JC) students are not able to score well in it. My current group of JC students in my GCE A-Level H2 Chemistry Tuition Classes can attest to it.
The only new Chemistry Concepts they learned here is a Mathematical Formulae to “Calculate the Molecular Formulae of Hydrocarbons using Combustion Data“. We will talk about this later in more details. Stay tuned!
Meanwhile, i would like to revise with you some
Key Definitions that you are expected to know:
* Mole
– is the unit of the amount of substance containing a number of particles equal to Avogadro constant
* Avogadro Constant
– states that there are 6.02 x 1023 particles in one mole of substance
* Empirical Formula
– Simplest formula which shows the ratio of the atoms of the different elements in a compound
* Molecular Formula
– Formula which shows the actual number of atoms of each element present in one molecule of a compound
* Excess Reagent
– Reactants that are in excess amount and are not consumed at the end of the chemical reaction
* Limiting Reagent
– Reactants that are completely consumed at the end of the reaction and determines the amount of product (s) formed
* Theoretical Yield
– Maximum amount of a product that can be obtained in a reaction from the given amount of reactants
* Actual Yield
– Actual amount of product obtained from the reaction and usually lesser due to incomplete reaction or product loss during reaction
* Molar Volume
– Volume occupied by 1 mole of Gas, which is 22.7 dm3 (at s.t.p) and 24.0 dm3 (at r.t.p)
Key Formulae you need to know are summarised below:
No. of Moles = Mass of Substance (g) / Molar Mass of Substance
No. of Moles of Gas = Volume of Gas / Molar Volume
Relative Atomic Mass = Σ (Isotopic Mass x Percentage Abundance) / 100
% Yield = (Actual Amount) / (Theoretical Amount ) x 100%
Concentration (g/dm3) = Mass of Solute (g) / Volume of Solution (dm3)
Molar Concentration (mol/dm3) = Moles of Solute (mol) / Volume of Solution (dm3)
In the next few posts, we will look at a large number of worked examples, particularly on Mole and Chemical Calculations. I will also give you some Quick Check Questions consisting of examination-type questions to practice what you learn. Stay Tuned!
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0.093 mol of Na+
Hi,
Would you please demonstrate how to calculate
– no. of ions / cations / anions
– no. of formula units
because I have difficulty of them?
I’m from Hong Kong
Hi Edward,
Welcome to this blog. Hong Kong is a nice place for education.
First you must know the meaning of IONS which are known as charged particles. These include both the positive charge ions (cations) and negatively charged ions (anions).
Example:
H2SO4(aq) -> 2H+(aq) + SO42-(aq)
As such, 1 mol of H2SO4 will dissociates in water to give 2 mol of H+ ions and 1 mol of SO42- ion.
So you will get 2 mol of positive ions and 1 mol of negative ions.
Hope this is clear.
PS: You might want to visit another website of mine that discussed Basic Chemistry principles, in order to build up your fundamentals. It is available at http://www.SimpleChemConcepts.com.
Cheers!
Sean