In the previous section, we completed the discussion on Subshell electronic configuration of elements. In this section we will see the the Mole concept.
First we will do an experiment. We will call it 'Experiment 1'
1. Take a long magnesium ribbon. Clean it well by scraping with a knife
2. Cut the ribbon into small pieces. Length of each piece must be less than 5 mm
3. Take 5 ml of dilute hydrochloric acid in a test tube
4. Put one piece of the ribbon into the acid
• Reaction takes place and hydrogen gas comes out as bubbles.
5. After some time, the formation of bubbles stops
• This is because, the reaction has come to an end. The magnesium piece is completely used up.
6. But what about the hydrochloric acid?. Is it completely used up?
• We can check this using a blue litmus paper. The blue litmus will turn red. This indicates that, some of the hydrochloric acid still remains.
■ We can conclude that, the reaction came to an end because, the magnesium, which is one of the reactants is completely used up.
2. Put the pieces of magnesium one by one into the acid
• We can see the bubbles of hydrogen forming
3. But after some time, the reaction stops.
• This time there is some magnesium left.
■ We can conclude that, the reaction came to an end because, the hydrochloric acid, which is one of the reactants is completely used up.
Then a question arises:
■ How much quantities should we take so that both the reactants are completely used up?
• The answer can be found by analysing the reaction between hydrogen and chlorine as an example:
1. The balanced chemical equation of this reaction is:
H2 + Cl2 → 2HCl
2. So, one molecule of hydrogen reacts with one molecule of chlorine to give two molecules of HCl. The symbolic illustration is shown in fig.10.1 below:
3. Based on fig.10.1, we can write the following:
• The hydrogen molecule has two atoms
• The chlorine molecule has two atoms
• One atom of hydrogen combines with one atom of chlorine, thus giving the first molecule of HCl
• The other atom of hydrogen combines with the other atom of chlorine, thus giving the second molecule of HCl
• The number of atoms of each element on both sides of the equation are equal. That is., the equation is balanced
Now consider fig.10.2 below:
• This time there are two molecules of hydrogen. So there are 4 atoms of hydrogen.
• When the reaction takes place, two atoms of hydrogen do not have any chlorine atoms to combine with.
• So, when the reaction is complete, one molecule of hydrogen will remain as such. That is., hydrogen, which is one of the reactants, is not completely used up.
Let us analyse some more instances like this:
■ Take one molecule of hydrogen and two molecules of chlorine
• In this case, two chlorine atoms do not have any hydrogen atoms to combine with
• So two atoms of chlorine (that is., one molecule of chlorine) will remain. The balanced equation is: H2 + 2Cl2 → 2HCl + Cl2
■ Take two molecules of hydrogen and two molecules of chlorine
• In this case, there are four atoms each for both hydrogen and chlorine.
So no atoms will remain. That is., all the atoms will be completely used up. The balanced equation is: 2H2 + 2Cl2 → 4HCl
■ Take three molecules of hydrogen and two molecules of chlorine
• In this case, two hydrogen atoms do not have any chlorine atoms to combine with
• So two atoms of hydrogen (that is., one molecule of hydrogen) will remain. The balanced equation is: 3H2 + 2Cl2 → 4HCl + H2
■ Take ten molecules of hydrogen and eight molecules of chlorine
• In this case, four hydrogen atoms do not have any chlorine atoms to combine with
• So four atoms of hydrogen (that is., two molecules of hydrogen) will remain. The balanced equation is: 10H2 + 8Cl2 → 16HCl + 2H2
■ Take twenty molecules of hydrogen and twenty molecules of chlorine
• In this case, there are forty atoms each for both hydrogen and chlorine.
• So no atoms will remain. That is., all the atoms will be completely used up. The balanced equation is: 20H2 + 20Cl2 → 40HCl
• So, for a complete reaction to take place, the ratio
Number of hydrogen molecules : Number of chlorine molecules
Must be equal to 1:1
• The nitrogen molecule has two atoms
• The three hydrogen molecules together have six atoms
• One atom of nitrogen combines with three atoms of hydrogen, thus giving the first molecule of ammonia
• The other atom of nitrogen combines with the remaining three atoms of hydrogen, thus giving the second molecule of ammonia
• The number of atoms of each element on both sides of the equation are equal. That is., the equation is balanced
• If we take 1 molecule of nitrogen, there must be 3 molecules of hydrogen.
• If we take 2 molecules of nitrogen, there must be 6 molecules of hydrogen.
• If we take 24 molecules of hydrogen, there must be 8 molecules of nitrogen.
■ So, in this case, for a complete reaction to take place, the ratio
Number of nitrogen molecules : Number of hydrogen molecules
Must be equal to 1:3
For the reaction between hydrogen and oxygen to give water, the balanced chemical equation is:
Number of hydrogen molecules : Number of oxygen molecules
Must be equal to 2:1
For the reaction between magnesium and oxygen to give magnesium oxide, the balanced chemical equation is:
2Mg + O2 → 2MgO■ So in this case, for a complete reaction to take place, the ratio
Number of magnesium molecules : Number of oxygen molecules
Must be equal to 2:1
So it is clear that, in any chemical reaction, we must take a specified number of molecules of each reactants. Other wise the reaction will not be completed
Now the next question arises:
• Molecules are very minute particles. How can we count them?
In the next section, we will find the answer this question.
First we will do an experiment. We will call it 'Experiment 1'
1. Take a long magnesium ribbon. Clean it well by scraping with a knife
2. Cut the ribbon into small pieces. Length of each piece must be less than 5 mm
3. Take 5 ml of dilute hydrochloric acid in a test tube
4. Put one piece of the ribbon into the acid
• Reaction takes place and hydrogen gas comes out as bubbles.
5. After some time, the formation of bubbles stops
• This is because, the reaction has come to an end. The magnesium piece is completely used up.
6. But what about the hydrochloric acid?. Is it completely used up?
• We can check this using a blue litmus paper. The blue litmus will turn red. This indicates that, some of the hydrochloric acid still remains.
■ We can conclude that, the reaction came to an end because, the magnesium, which is one of the reactants is completely used up.
Now we will do the same experiment in another manner. We will call it Experiment 2
1. Take just 1 ml of hydrochloric acid in a test tube.2. Put the pieces of magnesium one by one into the acid
• We can see the bubbles of hydrogen forming
3. But after some time, the reaction stops.
• This time there is some magnesium left.
■ We can conclude that, the reaction came to an end because, the hydrochloric acid, which is one of the reactants is completely used up.
From the two experiments, We can make a conclusion:
■ The reaction will come to an end when any one of the reactants is completely used upThen a question arises:
■ How much quantities should we take so that both the reactants are completely used up?
• The answer can be found by analysing the reaction between hydrogen and chlorine as an example:
1. The balanced chemical equation of this reaction is:
H2 + Cl2 → 2HCl
2. So, one molecule of hydrogen reacts with one molecule of chlorine to give two molecules of HCl. The symbolic illustration is shown in fig.10.1 below:
 |
| Fig.10.1 |
• The hydrogen molecule has two atoms
• The chlorine molecule has two atoms
• One atom of hydrogen combines with one atom of chlorine, thus giving the first molecule of HCl
• The other atom of hydrogen combines with the other atom of chlorine, thus giving the second molecule of HCl
• The number of atoms of each element on both sides of the equation are equal. That is., the equation is balanced
 |
| Fig.10.2 |
• When the reaction takes place, two atoms of hydrogen do not have any chlorine atoms to combine with.
• So, when the reaction is complete, one molecule of hydrogen will remain as such. That is., hydrogen, which is one of the reactants, is not completely used up.
■ Take one molecule of hydrogen and two molecules of chlorine
• In this case, two chlorine atoms do not have any hydrogen atoms to combine with
• So two atoms of chlorine (that is., one molecule of chlorine) will remain. The balanced equation is: H2 + 2Cl2 → 2HCl + Cl2
■ Take two molecules of hydrogen and two molecules of chlorine
• In this case, there are four atoms each for both hydrogen and chlorine.
So no atoms will remain. That is., all the atoms will be completely used up. The balanced equation is: 2H2 + 2Cl2 → 4HCl
■ Take three molecules of hydrogen and two molecules of chlorine
• In this case, two hydrogen atoms do not have any chlorine atoms to combine with
• So two atoms of hydrogen (that is., one molecule of hydrogen) will remain. The balanced equation is: 3H2 + 2Cl2 → 4HCl + H2
■ Take ten molecules of hydrogen and eight molecules of chlorine
• In this case, four hydrogen atoms do not have any chlorine atoms to combine with
• So four atoms of hydrogen (that is., two molecules of hydrogen) will remain. The balanced equation is: 10H2 + 8Cl2 → 16HCl + 2H2
■ Take twenty molecules of hydrogen and twenty molecules of chlorine
• In this case, there are forty atoms each for both hydrogen and chlorine.
• So no atoms will remain. That is., all the atoms will be completely used up. The balanced equation is: 20H2 + 20Cl2 → 40HCl
• It is clear that, if we take equal number of molecules of both hydrogen and chlorine, both the reactants will be completely used up.
■ When the number of molecules of hydrogen and chlorine are equal, we can write this:
Number of hydrogen molecules : Number of chlorine molecules = 1:1• So, for a complete reaction to take place, the ratio
Number of hydrogen molecules : Number of chlorine molecules
Must be equal to 1:1
But this ratio will not work for all chemical reactions. For example consider the manufacture of ammonia. The balanced chemical equation is:
N2 + 3H2 → 2NH3
From the equation, it is clear that:• The nitrogen molecule has two atoms
• The three hydrogen molecules together have six atoms
• One atom of nitrogen combines with three atoms of hydrogen, thus giving the first molecule of ammonia
• The other atom of nitrogen combines with the remaining three atoms of hydrogen, thus giving the second molecule of ammonia
• The number of atoms of each element on both sides of the equation are equal. That is., the equation is balanced
• If we take 1 molecule of nitrogen, there must be 3 molecules of hydrogen.
• If we take 2 molecules of nitrogen, there must be 6 molecules of hydrogen.
• If we take 24 molecules of hydrogen, there must be 8 molecules of nitrogen.
■ So, in this case, for a complete reaction to take place, the ratio
Number of nitrogen molecules : Number of hydrogen molecules
Must be equal to 1:3
2H2 + O2 → 2H2O
■ So in this case, for a complete reaction to take place, the ratioNumber of hydrogen molecules : Number of oxygen molecules
Must be equal to 2:1
2Mg + O2 → 2MgO■ So in this case, for a complete reaction to take place, the ratio
Number of magnesium molecules : Number of oxygen molecules
Must be equal to 2:1
Now the next question arises:
• Molecules are very minute particles. How can we count them?
In the next section, we will find the answer this question.
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