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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