CBSE Class 9 Science Practical Skills – Law of Conservation of Mass

CBSE Class 9 Science Practical Skills – Law of Conservation of Mass

EXPERIMENT

AIM
To verify the law of conservation of mass in a chemical reaction,

MATERIALS REQUIRED
Barium chloride (BaCl₂.2H₂0), sodium sulphate (Na₂SO₄.10H₂O), distilled water, two beakers (100 mL), one beaker (150 mL), physical balance, spring balance (0-500 g), polythene bag, two watch glasses and a glass rod.

THEORY

1. During a chemical reaction, the mass of the reactants is equal to the mass of the products.
2. Mass can neither be created nor be destroyed.
3. It can be demonstrated using a precipitation reaction between BaCl₂ (aq) and Na₂SO₄(aq).
4. The reaction involved is
   
   According to the law of conservation of mass,
   Mass of reactants (barium chloride + sodium sulphate) = Mass of products (barium sulphate+ sodium chloride).

PROCEDURE

1. Pour 50 mL distilled water in two 100 mL beakers.
2. Using the physical balance, weigh two watch glasses.
3. Weigh 3.6 g of BaCl₂.2H₂0 in a watch glass of known mass and dissolve it in a beaker containing 50 mL distilled water and label it ‘A’.
4. Similarly, weigh 8.05 g of Na₂SO₄.10H₂O in another watch glass of known mass and dissolve it in the other beaker containing 50 mL distilled water. Label it ‘B’.
5. Weigh the 150 mL beaker using spring balance and polythene. Label it ‘C’.
6. Mix solutions of beaker ‘A’ and ‘B’ together in beaker ‘C’. Mix the contents using a glass rod.
7. A white precipitate due tc the formation of barium sulphate (BaSO₄) appears in the beaker ‘C’.
8. Weigh the beaker containing the reaction mixture again to find out the mass of products formed.
9. Compare the masses of the contents before ana after the reaction.
   

OBSERVATIONS

1. Mass of 50 mL distilled water (assuming density as 1g /cc) = 50.0 g
2. Mass of BaCl₂.H₂0 = 3.6 g
3. Mass of BaCl₂ solution = 53.6 g
4. Mass of Na₂SO₄.10H₂O = 8.05 g
5. Mass of Na₂SO₄ solution =58.05 g
6. Total mass of reactants (solutions of BaCl₂ and Na₂SO₄) = 53.6 + 58.05 = 111.65 g
7. Mass of empty 150 mL beaker,m₁ =………………….g
8. Mass of reaction mixture before precipitation, m₂= m₁+ 111.65 g = ……………………g
9. Final mass of reaction mixture after precipitation, m₃ =……………………. g

RESULT

1. On comparing the mass of reactants (m₂) with the mass of products (m₃), we find m₂ equals m₃
   (within reasonable limits).
2. This proves the law of conservation of mass.

PRECAUTIONS

1. Accurate measurement of masses should be ensured.
2. The spring balance should be held vertical while taking measurements.
3. Before taking reading, ensure the pointer of the spring balance is at zero mark.
4. The reading of spring balance should be noted when its pointer comes to rest.
5. Use small amount of chemicals.
6. Mix solutions of BaCl₂ and Na₂SO₄ slowly with constant stirring.

INTERACTIVE SESSION

Examiner :
What is the aim of your experiment?
Examinee:
To prove the law of conservation of mass.

Examiner :
What is law of conservation of mass?
Examinee:
Law of conservation of mass means that during a chemical reaction, the mass of reactants and products remains the same.

Examiner :
In this experiment, instead of  BaCl₂  can you use CaCl₂? Why?
Examinee:
Yes, CaCl₂ can also be used, because on reaction with Na₂SO₄, a white precipitate of CaSO₄ will be formed.

Examiner :
Give the equation involved using CaCl₂ and  Na₂SO₄.
Examinee:

Examiner :
Can any other barium salt be used to carry out the reaction.
Examinee:
Yes, any other soluble barium salt can be used in the reaction.

Examiner :
Explain why is it so?
Examinee:
Basically this reaction can be represented as:

Since the reaction is between Ba²⁺ and S0₄^2- ions, therefore, any other soluble barium salt can be used for the reaction.

Examiner:
What type of a reaction is this?
Examinee:
It is a double displacement, precipitation reaction.

Examiner:
9.80 g of  KClO₃ on heating produces 3.84 g of oxygen and the residue KCl left behind weighs 5.92 g. Show that the result illustrates the law of conservation of mass.
Examinee:
Mass of reactant (m₁)=9.80 g
Mass of productt (m₂)= 3.84+5.92 =9.76 g
Difference m₂ – m₁  =0.04 g which may be an experimental error

Examiner:

Examinee:

Examiner :
Can a combination reaction also be used to prove law of conservation of mass?
Examinee:
Yes

Examiner :
Give an example.
Examinee:
The reaction between iron and sulphur to yield iron sulphide can be used to prove the law of conservation of mass.

Examiner :
Does the law of conservation of mass hold good for nuclear reactions?
Examinee:
No, it holds good only for chemical reactions.

Examiner :
Who gave the law of conservation of mass?
Examinee:
The French chemist Antoine Lavoisier gave the law of conservation of mass.

Examiner :
In nuclear reactions, why does law of conservation of mass not hold good?
Examinee:
Because in nuclear reactions, mass is converted into energy according to the Einstein equation
E =mc².

Examiner :
Does the law hold good for physical changes also?
Examinee:
Yes, it holds good for physical changes too.

Examiner :
Give an example to prove it,
Examinee:
Ice on melting gets converted into wider. If we melt known mass of ice in a flask (weight known), the mass of water obtained in the flask remains same, which proves the law of conservation of mass during a physical change.

NCERT LAB MANUAL QUESTION

Question 1:
What are the other precipitation reactions that can be conveniently studied in the laboratory to verify this law?
Answer:
Other precipitation reactions which can prove this law is a reaction between NaCl (aq) and AgN03(aq) as per the following equation:

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA

Question 1:
Which of the following reactions best illustrates the law of conservation of mass?

Question 2:
In the following reaction 2NaN₃ → 2Na + 3N₂ , if 750 g of NaN₃ decompose to form 265.20 g of Na, how much N₂  is produced?
(a) 484.80 g
(b) 576.80 g
(c) 357.80 g
(d) 464.80 g

Question 3:
On heating 20 g CaCO₃ , 8.8 g Co₂ is obtained. The mass of CaO formed is
(a) 11.2 g
(b) 10.9 g
(c) 10.2 g
(d) 11.9 g

Question 4:
The law of conservation of mass was formulated by
(a) Dalton
(b) Lavoisier
(c) Newton
(d) Boyle

Question 5:
In the given equation for the formation of aluminium sulphide by heating a mixture of aluminium and sulphur
2Al+ 3S → Al₂S₃
how many gram of sulphur is needed if 54 g aluminium reacts with it to form 150 g Al₂S₃? (mass of Al = 27 g, mass of S = 32 g)
(a) 64
(b) 04
(c) 76
(d) 96

Question 6:

(a) 46 g
(b) 44 g
(c) 64 g
(d) 54 g

Question 7:
When 4 g of hydrogen combines with 32 g of oxygen, 36 g of water is formed. This observation is in accordance with
(a) law of constant composition
(b) law of conservation of mass
(c) law of reciprocal proportions
(d) law of multiple proportions

Question 8:
How many grams of hydrogen are required to produce 68 grams of ammonia if 56 grams of nitrogen are taken initially?
(a) 12 g
(b) 24 g
(c) 32 g
(d) 124 g

Question 9:
In the reaction between sodium sulphate solution and barium chloride solution, the products formed are
(a) precipitate of barium sulphate and solution of sodium chloride
(b) solution of barium sulphate and precipitate of sodium chloride
(c) barium sulphate and sodium chloride are both in solution form
(d) barium sulphate and sodium chloride both form precipitates

Question 10:
The reaction between sodium sulphate and barium chloride is a type of
(a) combination reaction
(b) exothermic reaction
(c) precipitation reaction
(d) none of the above.

Question 11:
Law of conservation of mass states that
(a) Matter can be created but cannot be destroyed
(b) Matter cannot be created but can be destroyed by burning it
(c) Matter can neither be created nor be dest-royed
(d) Matter can be created as well as destroyed

Question 12:
In the process of reacting barium chloride with sodium sulphate, which of the following statement is wrong?
(а) The solutions should be weighed carefully
(б) The solutions should be mixed thoroughly
(c) The solutions should never be stirred after mixing
(d) The beaker should be weighed again after mixing

Question 13:
The colour of the precipitate formed on mixing sodium sulphate and barium chloride is
(a) black
(b) muddy brown
(c) white
(d) colourless

ANSWER KEY
Multiple Choice Questions/VSA

1. (b)
2. (a)
3. (a)
4. (b)
5. (d)
6. (b)
7. (a)
8. (a)
9. (a)
10. (c)
11. (c)
12. (c)
13. (c)

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