Gce O Level Chemistry : Moles

Stoichiometry and the Mole Concept

Mole Calculations is considered the No. 1 Killer Topic for GCE ‘O’ Levels / IGCSE / IB / IP Chemistry by students as well as educators (teachers, tutors) alike.  Many students cannot understand how Mathematical Calculations are being combined with abstract Chemistry Concepts, not to mention to score in examinations & tests with questions related to Mole Calculations.

What is a mole ?

A mole of an atom or molecule is equal to its atomic or molecular mass. Lets take the example of Calcium. The atomic mass of Calcium is 40 which means that 1 mole of Calcium will be equal to 40 grams. For gases like Hydrogen 1 mole will be equal to molecular mass as they are diatomic and exist as molecule. Which means that 1 mole of Hydrogen will be equal to 2 grams as it molecular mass is equal to 2. for Oxygen it will be equal to 32. But 1 mole of Oxygen atom is equal to 16 While that of hydrogen is equal to 1 .For Molecules Like water (H2O) 1 mole will be equal to its molecualr mass which can be calculated by adding atomic mass of hydrogen and Oxygen (1+1+16)=18. We have added 1 two times because there are two atoms of Hydrogen.
In short 1 mole of a substance is equal to its molecular or atomic mass.

What is an Avogadro number ?

The avogadro number is defined as the number of atom in 12g of Carbon 12 isotope. It is 6.02 multiplied by 10 to the power 23. 1 mole of every substance contain Avogadro number of Particles.

Molar Gas Volume

1 mole of every gas occupy the volume of 24000 centimetre cube or 24 dm cube at room temperature and pressure.

Calculation with Moles ?

Q1) How Many Moles are there in 64 grams of Oxygen
Ans) Number of Moles = mass in grams/atomic or molecular mass.
thus 64/32=2 moles
Q2) How many moles are present in 72 dm cube of Oxygen
Ans) Volume/Molar Gas Volume
Thus 72/24= 3 moles
Similarly a Question can be prepared for finding out the mass of an element by giving number of moles you just have to put the values inthere correct places in the formulae

Number of Moles=Mass in grams/atomic or Molecular mass



If volume is rquired than

Number of Moles=Volume of Gas/Molar Gas Voume

Now Solve a little harder Question

Q)What Volume 10 Grams of Hydrogen Occupy at room temeperature and pressure.

Ans)First find the number of moles in 8G of Hydrogen by dividing mass by atomic or molecular mass

10/2=5 moles

Now you know that 1 mole of Hydrogen occupy the space of 24dm cube. How much space will 5 moles occupy. simply multiply 5 by 24 and the answer is 120 dm cube.

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