NCERT Class 9 Science Lab Manual – Law of Conservation of Mass

NCERT Class 9 Science Lab Manual – Law of Conservation of Mass

EXPERIMENT

Aim
To verify the law of conservation of mass in a chemical reaction.

Theory

1. Matter can neither be created nor destroyed in a chemical change.
2. Law of conservation of mass: In a chemical reaction (except for nuclear reactions as matter can change to energy) mass can neither be created nor destroyed.
   The reactants —> atoms of one or more substances simply get rearranged to form product.
   Total mass of reactants before reaction = Total mass of the product after reaction
   This law was proposed by Antoine Lavoisier.
3. Chemicals undergo several kind of changes, i.e. Physical, Chemical and nuclear.
4. Physical change: Matter only changes its phase from solid to liquid and from liquid to gas.
   
5. Chemical change: The atoms of reactants rearrange and form new compounds.
   
6. Nuclear change: The nucleus of an atom can be changed by adding/reducing the number of protons, in it/by splitting of nucleus.
   For example, Uranium can undergo fission to become Barium and Krypton.
7. In chemical reaction, when the reactants react they may take/give heat, gas may be released, bubbles may be formed, colour may change or precipitate may be formed.

Materials Required
Two watch glasses, beakers, weighing balance and glass rod.

Chemicals Required
Distilled water and one of the following sets of chemicals.

METHOD A

Procedure

1. Take two watch glasses and weigh them using the physical balance and record their weight.
2. Weigh 3.6g of BaCl₂.2H₂O in a watch glass of known mass.
3. Take 100 ml beaker with 50 ml distilled water in it. Label it as Beaker ‘A’.
   
4. Dissolve the weighed BaCl₂.2H₂O in the beaker ‘A’.
5. Now weigh 8.05 g of Na₂SO₄.H₂O in another watch glass of known mass and dissolve it in the other beaker containing 50 ml of distilled water. Label the beaker as ‘B’.
6. Take another beaker. Weigh it and label it as ‘C’
7. Mix the contents of beaker ‘A’ and ‘B’ in beaker ‘C’ Stir it with glass rod.
8. You will observe the white precipitate that is formed in beaker ‘C’, due to the formation of (BaSO₄) Barium Sulphate.
9. Weigh beaker ‘C’ again with the product obtained and record your observations.
10. To get the mass of the product formed, subtract the mass of beaker ‘C’ before adding the solutions and after adding the solutions from ‘A’ and ‘B’ beaker.

Observations

1. Mass of Reactants = 44 g
2. Mass of Products = 44 g
3. Mass of BaCl₂.2H₂O = 3.6 g
4. Mass of  BaCl₂, solution = 53.6 g
5. Mass of Na₂SO₄.10H₂O = 8.05 g
6. Mass of Na₂SO₄ solution = 58.05 g
7. Total mass of reactants is 53.6 g + 58.05 g = 111.65 g
8. (BaCl₂, soln.) (Na₂SO₄soln.)
9. Mass of beaker ‘C’ is  C₁ = 500 g
10. Mass of reaction mixture on adding in beaker ‘C’ is C₂ = 611.65 g
11. Mass of the product formed = C₂ – C₁  = 111.65 g
    Note: Mass of 50 ml of distilled water = 50.0 g
    (density of water = 1 g/mL)

Result

1. Within the resonable experimental limits:-
2. Initial mass of reactant = Final mass of the product (reactant mixture)

METHOD B

Procedure

1. Prepare a 5% solution of any one pair of substances X and Y given.
2. Take a little amount of solution of Y in a conical flask and some solution of X in an ignition tube.
3. Hang the ignition tube in the flask carefully as shown in the figure. The solution from the test tube should not spill in the flask.
4. Put a cork on the flask.
5. Weigh the flask with its contents carefully.
6. Now tilt and swirl the flask gently, so that the solutions X and Y get mixed.
7. Weigh the conical flask again.
8. Record your observation.
9. Note the mass of the conical flask before reacti
   

Observations

1. Initial weight of conical flask + ignition tube with the respective solution X and Y = …….g
2. Weight of conical flask + ignition tube when solution X and Y are mixed and reaction has taken place =……. g.
3. The weight of reactants = Weight of products.

Result

1. Mass of the reactants = Mass of the products
2. There is no loss of mass in a chemical reaction.

Precautions

1. Be careful while using the weighing machine.
2. Prepare the solution in distilled water only.
3. Do not taste any chemical.
4. Fix a cork when solution X and Y are mixed.
5. Subtract the mass of conical flask + cork to get the resultant mass of product.
6. Cork should be used when the chemicals (reactants) are mixed, so that the gas, vapours formed is not allowed to escape.
7. The law is verified only in closed system.
8. Do not allow the chemical to spill from the ignition tube while recording the initial mass.

Chemical Reactions

VIVA VOCE

Question 1:
Who gave the laws of chemical combination?
Answer:
Antoine Lavoisier and Joseph L. Proust.

Question 2:
Who coined the term atom?
Answer:
John Dalton.

Question 3:
Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction. Who said this?
Answer:
John Dalton.

Question 4:
How do atoms combine to form compound?
Answer:
Atoms combine in the ratio of small whole numbers.

Question 5:
What is the unit to measure atomic radius?
Answer:
Nanometer. (1 nm = 10^-9 m)

Question 6:
Define atom.
Answer:
An atom is the smallest particle of the element that can exist independently and retain all its chemical properties.

Question 7:
Define molecule.
Answer:
A molecule is the smallest particle of an element or a compound that can exist independently under ordinary conditions. It shows all the properties of the substance. A molecule is a group of two or more atoms that are chemically bonded together.

Question 8:
What does chemical formula mean?
Answer:
A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.

Question 9:
The number of atoms of each combining element in a chemical reaction of reactant is
Answer:
Equal to the number of atoms of each combing element at the product side.

Question 10:
Define ion.
Answer:
The charged atom (due to loss/gain of electrons) is called ion.

PRACTICAL BASED QUESTIONS

Question 1:
In a chemical reaction of sodium carbonate (5.3 g) and ethanoic acid (6 g), it forms (8.2 g) sodium ethanoate, (2.2 g) carbon dioxide and (0.9 g) water.
Does this product formed follow the law of conservation of mass?
Answer:

Question 2:
Give the chemical formula of sodium carbonate and calcium hydroxide.
Answer:
Na₂CO₃ – sodium carbonate, Ca(OH)₂ – calcium hydroxide.

Question 3:
In a chemical reaction 0.096 g of boron combine with 0.144 g of oxygen to form a compound. Calculate the mass of compound formed.
Answer:
Boron + oxygen —> compound of Boron and Oxygen
0.096 g + 0.144 g —> 0.240 g.
This is due to the law of conservation of mass.

Question 4:
A student tried to verify the law of conservation of mass by reacting zinc with dil. sulphuric acid, but he could not verify its law of conservation. State the reason.
Answer:
The chemical reaction of zinc with dilute sulphuric acid produces H₂, gas which can easily escape, hence the precautions should be taken while calculating the mass of reactants and products in such cases.

Question 5:
Under what condition one can verify the law of conservation of mass?
Answer:
It can be verified only in a closed system.

NCERT LAB MANUAL QUESTIONS

Question 1:
What are the other precipitation reactions that can be conveniently studied in the laboratory to verify this law?
Answer:
The reaction of sodium carbonate Na₂CO₃. in water and calcium chloride (CaCl₂) gives the precipitate, this reaction can be used to verify the law of conservation of mass.
The chemical reaction that occurs is:
Na₂CO₃ (aq) + CaCl₂(aq) —> 2NaCl (aq) + CaCO₃ (s)

Question 2:
How can the law be verified by studying combination reaction? Suggest a procedure for the same.
Answer:
By taking the known quantity of reactants and reacting them to give the product whose mass can be measured we can use combination reaction also to verify the law of conservation of mass.

MULTIPLE CHOICE QUESTIONS (MCQs)
Questions based on Procedural and Manipulative Skills

Question 1:
The atomic mass of Ca is 40 u and another atomic mass of Ca is 44 u. Are these masses in agreement with Dalton’s atomic theory?
(a) Yes
(b) No
(c) Yes, these are two isotopes of Ca not discovered at that time
(d) No, because Dalton said that all atoms of same element has same mass.

Question 2:
The mass number of an atom that has 10 protons, 10 electrons and 11 neutrons is
(a) 31
(b) 20
(c) 21
(d) 30.

Question 3:
Give the formula for sulphurous acid.
(a) ^H₂^{S
(b) H}₂^SO₃
^{(c) H}₂^SO₄
^{(d) HSO}₃

Question 4:
Fe²⁺  ion is named as
(a) Iron (II) ion
(b)  Ferric (II) ion
(c) Ferrous (II) ion
(d) both (a) and (c)

Question 5:
In the given reaction:
2H₂ + O₂ — >  2H₂O
the law of conservation of mass is proved because
(a) atoms of hydrogen and oxygen in reactant is equal to the atoms in product.
(b) mass of hydrogen and oxygen atom in the reactant is equal to the mass of water in the product.
(c) atoms of hydrogen and oxygen combine to form water
(d) none of the above.

Question 6:
Which of the following chemists initially stated and studied the law of conservation of mass?
(a) Lavoisier
(b) Boyle
(c) Priestley
(d) Cavendish.

Question 7:
Balancing of chemidhl equation is based on
(a) Law of conservation of momentum
(b) Law of conservation of weight
(c) Law of conservation of mass
(d) Law of proportion.

Question 8:
Write the name of the compound that has 2 atoms of phosphorus and 5 atoms of oxygen.
(a) Phosphorus oxide
(b) Phosphorus trioxide
(c) Phosphorus pentoxide
(d) Phosphorus dioxide.

Question 9:
The law of conservation of mass states that:
(a) mass can either be created or destroyed
(b) mass can neither be created nor destroyed
(c) mass can be destroyed but not created
(d) mass can be created but not destroyed

Question 10:
36 g of water always contain 4 g of H₂ and 32 g of oxygen. Which statement of Dalton’s theory support the above statement?
(a) atoms cannot be created nor destroyed in a chemical reaction
(b) atoms combine chemically in definite ratios to form compounds
(c) the relative number and kinds of atoms are constant in a given compound
(d) atoms of a given element are identical in mass and chemical properties.

Question 11:
The law of conservation of mass was stated by:
(a) Dalton
(b) Louis Pasteur
(c) Neil Bohr
(d) Antoine L. Lavoisier

Question 12:
In a chemical reaction, the sum of masses of the reactants and products remains unchanged. This is known as
(a) Law of constant proportion .
(b) Law of multiple proportion
(c) Law of constant mass
(d) Law of conservation of mass

Question 13:
How many grams of sodium oxide is formed by burning 46g sodium in air according to the equation:
4Na + O₂ —> 2Na₂O
(a) 78g
(b) 87g
(c) 62g
(d) 94g

Question 14:
Balance the following equation so as to fulfill the law of conservation of mass
C₆H₁₂O₆ +….O_{2  —>} …CO₂+…H₂O
The correct value at blank space should be of:-
(A) 4
(b) 6
(c) 16
(d) 10

Questions based on Reporting and Interpretation Skills

Question 15:
Equal parts by mass of sulphur reacts with oxygen to form a product sulphur dioxide. If we take 2 parts of sulphur and react with 3 parts of oxygen the new compound formed will be
(a) S₂O₃
(b) S₃O₂
(c) SO₃
(d) SO₂.

Question 16:
Two chemicals A and B react completely to form C and D. If 4 g of A and 6 g of B react together and 3 g of C are produced. What is the amount, of D?
(a) 3 g
(b) -6g
(c) 7 g
(d) 5g

Question 17:
Pick the wrong choice.
(a)K⁺ ,  potassium ion
(b) Na²⁺, sodium ion
(c) O^2-, oxide ion
(d) Br^– , bromide ion

Question 18:
A compound has two atoms of sodium, three atoms of oxygen and one atom of carbon, its formula is
(a)  Na₃CO₂
(b) (NaCO)₂
(c) Na₂O₃C
(d) Na₃CO3.

Question 19:
In the chemical reaction of hydrogen and oxygen to form water,
2H2 + O₂ —> 2H₂O
the 4 grams of oxygen react with hydrogen to form 36 gm of water. The mass of hydrogen is
(a) 8 g
(b) 30 g
(c) 32 g
(d) 4 g.

Question 20:
The correct equation that shows the formation of ammonia gas is
(a) N₂ + H₂ -> NH3
(b) 2N₂ + 3H₂ -> 2NH3
(c) N₂ + 3H₂ -> 2NH3
(d) N₂ + 3H₂ -> 3NH3.

Question 21:
Iron reacts with water and oxygen to show rusting. The correct equation is
(a) Fe + O₂ + H₂O —> Fe₂O3.H₂O
(b) Fe + O₂ + xH₂O —> Fe₂O3.xH₂O
(c) 4Fe + 3O₂ + nH₂O —>2Fe₂O3.nH₂O
(d) 2Fe + 3O₂ + nH₂O—>2Fe₂O3.nH₂O.

Question 22:
The follow ing symbols represent one atom of different element.

Question 23:
In the given reaction
Na_xCO₃  + yHCl  —> 2NaCl + CO₂ + H₂O
the correct integer of x and y is
(a) x = 2, y = 4
(b) x = 2, y = 2
(c) x = 1, y = 2
(d) x = 2,y= 1.

Question 24:
24 g of magnesium burns in air to form magnesium oxide as shown below
2Mg + O₂ —> 2MgO
the magnesium oxide formed is
(a) 20 g
(b) 24 g
(c) 16 g
(d) 40 g.

Question 25:
In the given reaction
2Na + Cl₂ —>2NaCl
(46 g)     (75 g)    (95 g)
the amount of unreacted reactants in (g) is
(a) 23 g
(b) 20 g
(c) 26 g
(d) 25 g.

SCORING KEY WITH EXPLANATION

1. (d)All atoms of same element have same mass.
2. (c)Mass number = P + N, 10 + 11 = 21.
3. (b)The sulphurous acid have 1 oxygen atom less then sulphuric acid.
4. (d)It is ferrous/iron (II) ion.
5. (b)Law of conservation of mass in chemical reaction.
6. (a)Lavoisier studied the law of conservation of mass.
7. (c)The number of reacting atoms = same number of atoms of element in product.
8. (c)Pentoxide means 5 atoms of oxygen.
9. (b)Mass can neither be created nor destroyed as per the law of conservation of mass.
10. (a)The mass of reacting atoms = mass of product atoms.
11. (d) Lavoisier studied the law of conservation of mass. .
12. (d)Mass can neither be created nor destroyed as per the law of conservation of mass.
13. (c)(23 x 2) +16 = 62.
14. (b)The balanced chemical equation satisfies the laws of conservation of mass.
15. (c)
16. (c) A + B = C + D, 4 + 6 = 3 + 7.
17. (b) Sodium ion loses 1 electron and has 1+ charge on it.
18. (d) As per the number of atom given the formula of a compound is formed.
19. (c) A + B = C, 4 + 32 = 36.
20. (c) The number of reacting atoms = same number of atoms of element in product.
21. (c) It is the correct balanced equation.
22. (c) The number of reacting atoms = same number of atoms of element in product.
23. (b) The number of reacting atoms = same number of atoms of element in product.
24. (d) 24 + 16 = 40g.
25. (c) Unreacted reactants = (46 + 75) – 95 = 26g

 

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