The chapter deals with conservation of mass, constant proportions, Dalton’s atomic theory, covalent and ionic bonding, chemical formulae, and molecular/formula-unit mass.
A. THINK IT OVER
Q1. Water can be obtained from various sources. Are all these samples of water chemically identical?
Answer:
Pure water obtained from different sources is chemically identical. It is always made of hydrogen and oxygen and has the formula H₂O.
However, natural water from rivers, wells or oceans may contain different dissolved salts and other impurities. After purification, the water is chemically the same.
Q2. Oxygen is sometimes represented as O and sometimes as O₂. What is the difference between these symbols?
Answer:
- O represents one atom of oxygen.
- O₂ represents one molecule of oxygen containing two oxygen atoms joined together.
Q3. Why does dissolved salt in water conduct electricity, but sugar does not?
Answer:
Salt forms charged particles called ions when dissolved in water. These ions can move and carry electric current.
Sugar dissolves in water but does not form ions. Therefore, a sugar solution does not conduct electricity.
B. ACTIVITY 9.1 - LET US INVESTIGATE A PHYSICAL CHANGE
The activity asks us to weigh water and salt before and after the salt dissolves. The purpose is to check whether mass changes during a physical change.
Activity in simple words
- Take a clean beaker and place it on a weighing balance.
- Add about 50 mL water.
- Add some common salt.
- Note the total mass.
- Stir or swirl until the salt dissolves.
- Weigh it again.
Q1. What do you observe?
Answer:
The mass of the salt solution after dissolving is equal to the total mass of the water and salt before dissolving.
Therefore,
Mass before dissolving = Mass after dissolving
There is practically no change in mass during this physical change.
Q2. If a piece of paper is weighed before and after tearing it into pieces, will its mass change?
Answer:
No. Its mass will remain the same because tearing paper changes only its size and shape. No matter is lost or formed.
C. ACTIVITY 9.2 - LET US INVESTIGATE A CHEMICAL CHANGE
In this activity, vinegar reacts with baking soda. Carbon dioxide gas is formed. Two different experimental arrangements are used to understand why a chemical reaction may sometimes appear to lose mass.
Experimental Set-up 1
Activity in simple words
Vinegar is placed in a flask and baking soda is kept separately in a balloon. Their total mass is measured. Then the baking soda is added to the vinegar. The reaction takes place in an open flask and carbon dioxide escapes.
Q1. What do you observe?
Answer:
Brisk bubbling or effervescence is seen because carbon dioxide gas is produced.
Q2. Are the initial and final readings the same?
Answer:
No. The final reading is lower than the initial reading.
Q3. A brisk effervescence is observed. The final reading does not match the initial reading. What can be the reason for this?
Answer:
Carbon dioxide gas is produced during the reaction. Since the flask is open, some carbon dioxide escapes into the air.
Therefore, the mass measured after the reaction appears to be lower.
Experimental Set-up 2
Activity in simple words
This time the balloon is fixed tightly to the mouth of the flask before the baking soda is mixed with vinegar. Therefore, the carbon dioxide produced cannot escape. It remains inside the flask and balloon.
Q1. What do you observe?
Answer:
Brisk effervescence takes place and the balloon inflates because carbon dioxide gas is produced.
Q2. Are the initial and final readings the same in this case?
Answer:
Yes. The initial and final masses are almost the same.
The carbon dioxide gas cannot escape because it is trapped inside the balloon.
Therefore,
Total mass before reaction = Total mass after reaction.
This proves the Law of Conservation of Mass.
D. ACTIVITY 9.3 - LET US VERIFY THE LAW OF CONSERVATION OF MASS
Activity in simple words
Two solutions are taken:
- sodium sulfate solution
- barium chloride solution
Their total mass is first measured separately. Then they are mixed and weighed again.
Q1. What do you observe when sodium sulfate solution and barium chloride solution are mixed?
Answer:
A white precipitate of barium sulfate is formed.
The reaction is:
Sodium sulfate + Barium chloride → Barium sulfate + Sodium chloride
Q2. Do you observe any change in the reading after mixing the solutions?
Answer:
No. The total mass remains the same.
Therefore,
Mass of reactants = Mass of products
This verifies the Law of Conservation of Mass.
E. THINK AS A SCIENTIST
Q. You are given a chemical reaction in which zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen
Design and perform an experiment to test the hypothesis that mass is conserved during the chemical reaction. You may use a set-up different from the one shown in Activity 9.2.
Answer:
Procedure
- Take some dilute hydrochloric acid in a conical flask.
- Put some zinc pieces inside a balloon without allowing them to fall into the acid.
- Fix the balloon tightly over the mouth of the flask.
- Weigh the complete flask and balloon. Note the initial mass.
- Lift the balloon so that the zinc pieces fall into the acid.
- Zinc reacts with hydrochloric acid and hydrogen gas is produced.
- The hydrogen gas collects inside the balloon.
- After the reaction is complete, weigh the complete arrangement again.
Observation
The mass before and after the reaction will be the same.
Conclusion
The hydrogen gas has not escaped. Therefore,
Mass of reactants = Mass of products
Hence, the Law of Conservation of Mass is obeyed.
F. EXAMPLE 9.1
Q. In a group activity, students place 4.0 g of calcium carbonate with 2.92 g of hydrochloric acid in a closed container. After the reaction is over, they measured 1.76 g of carbon dioxide, 0.72 g of water, and 4.44 g of calcium chloride. Verify whether the Law of Conservation of Mass is obeyed or not.
Answer:
Mass of calcium carbonate = 4.0 g
Mass of hydrochloric acid = 2.92 g
Total mass of reactants:
4.0 + 2.92 = 6.92 g
Mass of carbon dioxide = 1.76 g
Mass of water = 0.72 g
Mass of calcium chloride = 4.44 g
Total mass of products:
1.76 + 0.72 + 4.44 = 6.92 g
Therefore,
Mass of reactants = Mass of products = 6.92 g
Hence, the Law of Conservation of Mass is obeyed.
G. EXAMPLE 9.2
Q. 12 g of carbon combines with 32 g of oxygen to form 44 g of carbon dioxide. If 2.4 g of carbon reacts completely with oxygen, how much carbon dioxide will be produced?
Answer:
12 g carbon produces = 44 g carbon dioxide
Therefore,
1 g carbon produces:
44 ÷ 12 g carbon dioxide
So, 2.4 g carbon produces:
(44 ÷ 12) × 2.4
= 8.8 g
Answer:
8.8 g of carbon dioxide will be produced.
H. PAUSE AND PONDER - QUESTIONS 1-2
Q1. A student burns 10 g of ethanol in an open beaker. After the reaction, no residue is left in the beaker. Does this mean the Law of Conservation of Mass is violated? Explain.
Answer:
No. The Law of Conservation of Mass is not violated.
Ethanol burns and forms gaseous products such as carbon dioxide and water vapour. These gases escape into the air because the beaker is open.
If all the products were collected and weighed, their total mass would equal the mass of the substances that reacted.
Q2. When 20 g of hydrogen reacts completely with 160 g of oxygen, how much water is formed according to the Law of Conservation of Mass?
Answer:
Mass of hydrogen = 20 g
Mass of oxygen = 160 g
According to the Law of Conservation of Mass:
Mass of water = 20 + 160
= 180 g
Answer:
180 g of water is formed.
I. LAW OF CONSTANT PROPORTIONS
A pure compound always contains its elements in the same fixed ratio by mass. For example, pure water always contains hydrogen and oxygen in the mass ratio 1 : 8.
EXAMPLE 9.3
Q. Sodium chloride (NaCl) contains sodium and chlorine in the mass ratio of 23 : 35.5. If 46 g of sodium reacts completely, how much chlorine is needed to form NaCl?
Answer:
23 g sodium requires = 35.5 g chlorine
46 g sodium requires:
(35.5 ÷ 23) × 46
= 71 g
Answer:
71 g of chlorine is required.
J. PAUSE AND PONDER - QUESTIONS 3-6
Q3. A compound consists of 40% sulfur and 60% oxygen by mass. In a sample of the same compound containing 20 g of sulfur, what mass of oxygen must be present to satisfy the Law of Constant Proportions?
Answer:
Sulfur : Oxygen = 40 : 60
= 2 : 3
If sulfur = 20 g,
Oxygen:
20 × 3 ÷ 2 = 30 g
Answer:
30 g of oxygen must be present.
Q4. Carbon monoxide (CO) contains carbon and oxygen in the mass ratio of 3 : 4. How much oxygen will combine with 9 g of carbon to form carbon monoxide?
Answer:
Carbon : Oxygen = 3 : 4
For 9 g carbon:
Oxygen = 9 × 4 ÷ 3
= 12 g
Answer:
12 g of oxygen is required.
Q5. The Law of Definite Proportions holds true for compounds but not for mixtures. Give reason.
Answer:
A compound always contains its elements in a fixed ratio by mass.
A mixture can contain its substances in any proportion.
For example, water always has hydrogen and oxygen in a fixed ratio, but salt and water can be mixed in different amounts.
Q6. Students X and Y both prepared an oxide of copper by combining copper and oxygen in the ratios of 4 : 1 and 8 : 2, respectively. Do their results justify the Law of Constant Proportions? Explain.
Answer:
Yes.
Student X:
Copper : Oxygen = 4 : 1
Student Y:
8 : 2 = 4 : 1
Both have the same ratio of 4 : 1.
Therefore, their results support the Law of Constant Proportions.
K. WHAT IF...
Q. What if atoms could combine in any ratio and not in a fixed ratio? How would this affect the substances around us?
Answer:
If atoms could combine in any ratio, compounds would not have a fixed composition.
The same substance could have different properties at different times. The substances around us would become difficult to identify and predict.
L. DALTON'S ATOMIC THEORY - PAUSE AND PONDER
Dalton stated that atoms combine in simple whole-number ratios and that atoms are not created or destroyed during ordinary chemical reactions.
Q7. Assertion (A): 2 g of hydrogen combines with 16 g of oxygen to form 18 g of water.
Reason (R): According to Dalton’s Atomic Theory, atoms combine in a simple whole number ratio by mass to form compounds.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
Answer:
(iii) A is true, but R is false.
Explanation: The assertion is correct because:
2 g hydrogen + 16 g oxygen = 18 g water.
But Dalton's theory says that atoms combine in simple whole-number ratios, not that their masses combine in simple whole-number ratios.
M. IN-TEXT QUESTION - MOLECULES OF COMPOUNDS
Q. What happens if atoms of two different elements combine?
Answer:
Atoms of two different elements can combine to form a molecule of a compound.
For example, hydrogen and chlorine combine to form hydrogen chloride, HCl.
N. PAUSE AND PONDER - QUESTIONS 8-9
Q8. Nitrogen has five valence electrons. Draw the structure of the nitrogen molecule (N₂).
Answer:
Each nitrogen atom has five valence electrons and needs three more electrons to complete its octet.
Two nitrogen atoms share three pairs of electrons.
The structure is:
N ≡ N
The two nitrogen atoms are joined by a triple covalent bond.
Q9. The atomic number of fluorine is 9. Explain the formation of the fluorine molecule (F₂).
Answer:
Electronic configuration of fluorine = 2, 7
Each fluorine atom needs one electron to complete its octet.
Two fluorine atoms share one electron each and form one shared pair.
F-F
Therefore, F₂ contains a single covalent bond.
O. IN-TEXT QUESTION - FORMATION OF WATER
Q. Hydrogen needs only one electron, while oxygen needs two electrons to acquire stable electronic configurations. How can oxygen share its two electrons with another atom that requires only one electron?
Answer:
One oxygen atom shares one electron each with two different hydrogen atoms.
Thus:
H-O-H
Each hydrogen gets two electrons in its K-shell and oxygen gets eight electrons in its outer shell.
The molecule formed is H₂O.
P. PAUSE AND PONDER - QUESTIONS 10-11
Q10. Show the formation of the following molecules:
(i) Carbon dioxide (CO₂)
Answer:
Carbon has four valence electrons and oxygen has six.
Carbon shares two pairs of electrons with each oxygen atom.
Structure:
O = C = O
There are two double covalent bonds.
(ii) Hydrogen sulfide (H₂S)
Answer:
Sulfur has six valence electrons and requires two more.
It shares one electron each with two hydrogen atoms.
Structure:
H-S-H
(iii) Ammonia (NH₃)
Answer:
Nitrogen has five valence electrons and needs three more.
It shares one electron each with three hydrogen atoms.
Nitrogen forms three single covalent bonds.
Q11. Neon (atomic number 10) neither transfers nor shares its valence electrons. Explain.
Answer:
Electronic configuration of neon is 2, 8.
Its outermost shell already has eight electrons. Therefore, neon has a stable electronic configuration.
It does not need to gain, lose or share electrons.
Q. IN-TEXT QUESTIONS - IONIC BONDING
Q1. Identify four elements among the first 18 elements whose atoms can donate valence electrons to become stable.
Answer:
Examples are:
- Lithium - Li
- Beryllium - Be
- Sodium - Na
- Magnesium - Mg
These atoms can lose their valence electrons and form positive ions.
Q2. Will a sodium atom still be neutral after losing one electron? If not, what charge would it carry and why?
Answer:
No.
A sodium atom has 11 protons and 11 electrons.
After losing one electron, it has:
- 11 protons
- 10 electrons
Therefore, it has one extra positive charge and becomes Na⁺.
R. WHAT IF...
Q. What if we could see atoms directly? How would it help scientists and what challenges would it cause?
Answer:
If scientists could see atoms directly, they could understand more clearly how atoms are arranged and how chemical reactions take place.
However, atoms are extremely small, so observing and studying such tiny particles would require very advanced instruments and techniques.
S. PAUSE AND PONDER - QUESTIONS 12-15
Q12. What kind of ion will oxygen (O) form?
Answer:
Oxygen has six valence electrons. It gains two electrons to complete its octet.
Therefore, it forms a negatively charged ion:
O²⁻
It is called an oxide ion.
Q13. Fill in the blanks.
Among magnesium and chlorine, magnesium atom can give two electrons to become Mg²⁺. However, chlorine can take only one electron to become ____________. Now, __________ ion of magnesium and __________ ions of chlorine combine to give magnesium chloride.
Answer:
Chlorine becomes Cl⁻.
One Mg²⁺ ion and two Cl⁻ ions combine to form magnesium chloride.
Therefore:
Mg²⁺ + 2Cl⁻ → MgCl₂
Q14. Show the formation of cations of potassium (K) and calcium (Ca) atoms, and the formation of their corresponding chlorides using diagrams.
Answer:
Potassium
Electronic configuration of K = 2, 8, 8, 1
It loses one electron:
K → K⁺ + e⁻
K⁺ configuration = 2, 8, 8
Chlorine gains this electron:
Cl + e⁻ → Cl⁻
Then:
K⁺ + Cl⁻ → KCl
Therefore, potassium chloride is KCl.
Calcium
Electronic configuration of Ca = 2, 8, 8, 2
It loses two electrons:
Ca → Ca²⁺ + 2e⁻
Two chlorine atoms each accept one electron:
2Cl + 2e⁻ → 2Cl⁻
Then:
Ca²⁺ + 2Cl⁻ → CaCl₂
Therefore, calcium chloride is CaCl₂.
Q15. Illustrate how sodium sulfide (Na₂S) is formed.
Answer:
Sodium has one valence electron. Sulfur has six valence electrons and needs two more.
Two sodium atoms each lose one electron:
2Na → 2Na⁺ + 2e⁻
Sulfur gains the two electrons:
S + 2e⁻ → S²⁻
Then:
2Na⁺ + S²⁻ → Na₂S
Therefore, the formula of sodium sulfide is Na₂S.
T. PAUSE AND PONDER - QUESTIONS 16-18
Q16. Name the following:
(i) CO₂
Answer:
Carbon dioxide
(ii) NO₂
Answer:
Nitrogen dioxide
(iii) SF₆
Answer:
Sulfur hexafluoride
(iv) PCl₃
Answer:
Phosphorus trichloride
Q17. Write the formula for the following:
(i) Sodium hydrogencarbonate
Answer:
NaHCO₃
(ii) Sulfur dioxide
Answer:
SO₂
(iii) Ferric chloride
Answer:
FeCl₃
(iv) Cuprous oxide
Answer:
Cu₂O
Q18. Write the formulae for the compounds formed from the following pairs of ions:
(i) Fe³⁺ and OH⁻
Answer:
Fe(OH)₃
(ii) K⁺ and CO₃²⁻
Answer:
K₂CO₃
U. ACTIVITY 9.4 - LET US EXPERIMENT
This activity compares ionic and covalent compounds by testing their solubility and electrical conductivity.
Activity in simple words
Samples such as camphor, sodium chloride, copper sulfate, sugar and naphthalene are tested to find:
- whether they dissolve in water,
- whether they dissolve in kerosene,
- whether they dissolve in petrol,
- whether they conduct electricity as solids,
- whether their water solutions conduct electricity.
Expected observations
| Compound | Water | Kerosene/Petrol | Conducts as solid? | Conducts in water? |
|---|---|---|---|---|
| Camphor | Insoluble | Soluble | No | No |
| Sodium chloride | Soluble | Insoluble | No | Yes |
| Copper sulfate | Soluble | Insoluble | No | Yes |
| Sugar | Soluble | Generally insoluble | No | No |
| Naphthalene | Insoluble | Soluble | No | No |
The result for “any other” compound depends on the compound chosen.
Q1. Group the compounds showing similar properties listed in Table 9.2.
Answer:
Ionic compounds:
- Sodium chloride
- Copper sulfate
They are generally soluble in water and conduct electricity when dissolved in water.
Covalent compounds:
- Camphor
- Sugar
- Naphthalene
They do not conduct electricity.
Q2. Camphor, naphthalene and other covalent compounds do not conduct electricity. Can you give a reason?
Answer:
Covalent compounds do not form freely moving ions.
Since there are no charged particles free to move, they cannot carry electric current.
Q3. Predict whether ionic and covalent compounds would conduct electricity in the molten state.
Answer:
- Ionic compounds: Yes. When melted, their ions become free to move and carry electric current.
- Covalent compounds: Generally no, because they do not contain freely moving ions.
The chapter explains that ionic compounds conduct when their ions are free to move.
V. PAUSE AND PONDER - QUESTIONS 19-20
Q19. What type of chemical bond is present in a solid compound that does not conduct electricity in the solid state but conducts electricity when dissolved in water?
Answer:
It has an ionic bond.
In the solid state, the ions are fixed in position. In water, the ions become free to move and can conduct electricity.
Q20. Metal M, with two electrons in its valence shell (M shell), reacts with oxygen to form a compound that is slightly soluble in water. Predict its:
(i) Formula
Answer:
The metal is magnesium with configuration 2, 8, 2.
It forms Mg²⁺ while oxygen forms O²⁻.
Therefore, the formula is MgO.
(ii) Type of bond
Answer:
Ionic bond
(iii) Electrical conductivity of its aqueous solution
Answer:
Its aqueous solution will conduct electricity because it contains ions that can move through the solution.
W. EXAMPLE 9.4
Q. Find the molecular mass of water (H₂O).
Given:
H = 1 u
O = 16 u
Answer:
Molecular mass of H₂O:
= (2 × 1) + (1 × 16)
= 2 + 16
= 18 u
X. EXAMPLE 9.5
Q. Find the molecular mass of carbon dioxide (CO₂).
Given:
C = 12 u
O = 16 u
Answer:
Molecular mass of CO₂:
= (1 × 12) + (2 × 16)
= 12 + 32
= 44 u
Y. PAUSE AND PONDER - QUESTIONS 21-22
Q21. Find the molecular mass of nitric acid (HNO₃).
Atomic masses:
H = 1 u
N = 14 u
O = 16 u
Answer:
Molecular mass of HNO₃:
= 1 + 14 + (3 × 16)
= 1 + 14 + 48
= 63 u
Q22. Find the molecular mass of methane (CH₄).
Atomic masses:
C = 12 u
H = 1 u
Answer:
Molecular mass of CH₄:
= 12 + (4 × 1)
= 16 u
Z. EXAMPLE 9.6
Q. Find the formula unit mass of sodium oxide (Na₂O).
Atomic masses:
Na = 23 u
O = 16 u
Answer:
Formula unit mass of Na₂O:
= (2 × 23) + 16
= 46 + 16
= 62 u
AA. EXAMPLE 9.7
Q. Find the formula unit mass of calcium nitrate, Ca(NO₃)₂.
Atomic masses:
Ca = 40 u
N = 14 u
O = 16 u
Answer:
Formula unit mass:
= 40 + 2[14 + (3 × 16)]
= 40 + 2(14 + 48)
= 40 + 124
= 164 u
AB. PAUSE AND PONDER - QUESTIONS 23-24
Q23. Find the formula unit mass of potassium chloride (KCl).
Atomic masses:
K = 39 u
Cl = 35.5 u
Answer:
Formula unit mass of KCl:
= 39 + 35.5
= 74.5 u
Q24. Find the formula unit mass of magnesium hydroxide, Mg(OH)₂.
Atomic masses:
Mg = 24 u
O = 16 u
H = 1 u
Answer:
Formula unit mass:
= 24 + 2(16 + 1)
= 24 + 34
= 58 u
END-OF-CHAPTER EXERCISE - REVISE, REFLECT, REFINE
The following questions are the chapter-end exercises.
Q1. A particular element (A) has one electron in its third shell. There is another element (B) with six electrons in its second shell.
(i) How many electrons does A tend to give or take to become stable?
Answer:
A will give one electron to become stable.
(ii) What kind of ion would it form?
Answer:
It will form a positive ion or cation, A⁺.
(iii) How many electrons does B tend to give or take to become stable?
Answer:
B will take two electrons to complete its octet.
(iv) What kind of ion would it form?
Answer:
It will form a negative ion or anion, B²⁻.
(v) If A and B were to combine, what kind of bond would be formed?
Answer:
An ionic bond would be formed because electrons are transferred from A to B.
(vi) What would be the formula for the compound thus formed?
Answer:
Two A⁺ ions are needed to balance one B²⁻ ion.
Therefore, the formula is:
A₂B
Q2. An element X has six electrons in its outer shell and forms a diatomic molecule.
(i) Why would that be so?
Answer:
X needs two more electrons to complete its octet. Two X atoms share two electrons each and become stable.
(ii) What kind of bond would it form?
Answer:
It would form a double covalent bond.
(iii) Draw the structure of the molecule it would form.
Answer:
X = X
(iv) A certain other element Y has two electrons in its second shell. Draw the structure of the molecule that X would form with Y.
Answer:
If Y has two electrons in its second shell, that shell is complete and Y is stable.
Therefore, Y normally does not combine with X to form a molecule.
Q3. You want to design a new ionic compound, where the total positive charge is 6+ and the total negative charge is 6-. Which of the following combinations gives the correct number of ions?
(i) 2 Al³⁺ and 3 Cl⁻
(ii) 3 Mg²⁺ and 1 PO₄³⁻
(iii) 2 Fe³⁺ and 3 O²⁻
(iv) 3 Ca²⁺ and 2 SO₄²⁻
Answer:
(iii) 2 Fe³⁺ and 3 O²⁻
Positive charge:
2 × (+3) = +6
Negative charge:
3 × (-2) = -6
The charges balance.
Q4. Choose the correct statement(s) and correct the false statement(s).
(i) Elements are made up of molecules and compounds are made up of atoms.
Answer:
False.
Correct statement:
Elements are made up of atoms. Some elements may exist as molecules. Compounds are made of atoms of different elements chemically combined.
(ii) The molecule of a compound is always made up of two or more atoms of the same kind.
Answer:
False.
Correct statement:
A molecule of a compound contains atoms of two or more different elements chemically combined.
(iii) One molecule of nitrogen gas contains three nitrogen atoms.
Answer:
False.
Correct statement:
One molecule of nitrogen gas contains two nitrogen atoms and is written as N₂.
(iv) Water is made of two hydrogen atoms, covalently bonded with one oxygen atom.
Answer:
True.
Water is H₂O.
Q5. Write the chemical formulae for the following compounds.
(i) Aluminium nitrate
Answer:
Al(NO₃)₃
(ii) Calcium oxide
Answer:
CaO
(iii) Ferric oxide
Answer:
Fe₂O₃
Q6. Write the formulae of the compounds formed from the following pairs of ions.
(i) Ca²⁺ and Br⁻
Answer:
CaBr₂
(ii) Al³⁺ and CO₃²⁻
Answer:
Al₂(CO₃)₃
(iii) K⁺ and SO₄²⁻
Answer:
K₂SO₄
(iv) NH₄⁺ and Cl⁻
Answer:
NH₄Cl
Q7. Which of the following, in Fig. 9.18, correctly represents Cl⁻ ion? Atomic number of chlorine = 17.
Options: (i), (ii), (iii), (iv) as shown in Fig. 9.18.
Answer:
Option (ii).
Explanation: Chlorine has 17 electrons:
2, 8, 7
A chloride ion, Cl⁻, gains one electron.
Therefore:
Cl⁻ = 18 electrons = 2, 8, 8
The diagram in option (ii) shows this arrangement.
Q8. Determine the formula unit mass of the following substances.
(i) Ammonium nitrate (NH₄NO₃), used as a nitrogen fertiliser, which is essential for plant growth.
Atomic masses:
N = 14 u
H = 1 u
O = 16 u
Answer:
NH₄NO₃ contains:
- 2 nitrogen atoms
- 4 hydrogen atoms
- 3 oxygen atoms
Formula unit mass:
= (2 × 14) + (4 × 1) + (3 × 16)
= 28 + 4 + 48
= 80 u
(ii) Phosphoric acid (H₃PO₄), used to make phosphate fertiliser and detergents.
Atomic masses:
H = 1 u
P = 31 u
O = 16 u
Answer:
= (3 × 1) + 31 + (4 × 16)
= 3 + 31 + 64
= 98 u
(iii) Sodium hydrogencarbonate (NaHCO₃), used to relieve acidity and helps in digestion.
Atomic masses:
Na = 23 u
H = 1 u
C = 12 u
O = 16 u
Answer:
= 23 + 1 + 12 + (3 × 16)
= 23 + 1 + 12 + 48
= 84 u
Q9. Write the formulae for the compounds formed by the reaction of:
(i) Magnesium and nitrogen
Answer:
Mg₃N₂
(ii) Lithium and nitrogen
Answer:
Li₃N
(iii) Sodium and sulfur
Answer:
Na₂S
(iv) Aluminium and oxygen
Answer:
Al₂O₃
Q10. Complete Table 9.3 by writing the formulae of the compounds formed by the cations on the left and the anions at the top. LiNO₃ is given as an example.
Answer:
| Cation ↓ / Anion → | NO₃⁻ | SO₄²⁻ | PO₄³⁻ |
|---|---|---|---|
| NH₄⁺ | NH₄NO₃ | (NH₄)₂SO₄ | (NH₄)₃PO₄ |
| Li⁺ | LiNO₃ | Li₂SO₄ | Li₃PO₄ |
| Al³⁺ | Al(NO₃)₃ | Al₂(SO₄)₃ | AlPO₄ |
| Cu²⁺ | Cu(NO₃)₂ | CuSO₄ | Cu₃(PO₄)₂ |
Q11. 5.3 g of sodium carbonate and 6.0 g of acetic acid react to produce 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Verify whether the Law of Conservation of Mass is valid.
Answer:
Total mass of reactants:
= 5.3 + 6.0
= 11.3 g
Total mass of products:
= 2.2 + 0.9 + 8.2
= 11.3 g
Therefore:
Mass of reactants = Mass of products
Hence, the Law of Conservation of Mass is valid.
Q12. If a species has 11 protons, 12 neutrons and 10 electrons, then:
(i) What is its atomic number and mass number?
Answer:
Atomic number = Number of protons
= 11
Mass number:
= Protons + Neutrons
= 11 + 12
= 23
(ii) Is it neutral, a cation or an anion? Explain.
Answer:
It is a cation.
It has:
- 11 protons
- 10 electrons
Therefore, it has one more positive charge than negative charge.
Its charge is +1.
(iii) Write its electronic configuration.
Answer:
It has 10 electrons.
Electronic configuration:
2, 8
(iv) Name the species.
Answer:
Atomic number 11 belongs to sodium.
Therefore, the species is a sodium ion, Na⁺.
Q13. Two elements, A and B, have the following configurations:
A: 2, 8, 5
B: 2, 8, 7
(i) Which element is more reactive?
Answer:
B is more reactive.
B needs only one electron to complete its octet, while A needs three electrons.
(ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing.
Answer:
They will form covalent bonds.
Both A and B need electrons. Neither easily gives electrons to the other. Therefore, they become stable by sharing electrons.
(iii) Predict the formula of the compound they would form.
Answer:
A requires three electrons and each B atom requires one electron.
Therefore, one A atom combines with three B atoms.
Formula = AB₃
Q14. Assertion (A): Copper sulfate conducts electricity in the molten state but not in the solid state.
Reason (R): Copper and sulfate ions are fixed in the lattice in molten state, while in solid state they can move freely.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.
Answer:
(iii) A is true, but R is false.
Explanation: In solid copper sulfate, the ions are fixed and cannot move freely.
In the molten state, the ions become free to move and can conduct electricity.
The reason given in the question has these conditions reversed.
Q15. The species ²⁷Al, ⁸⁰Br⁻ and ²⁰¹Hg²⁺ have 13, 35 and 80 protons, respectively. How many electrons and neutrons do they have?
Answer:
(i) ²⁷Al
Protons = 13
No charge is shown, so:
Electrons = 13
Neutrons:
= 27 - 13
= 14
Answer:
13 electrons and 14 neutrons
(ii) ⁸⁰Br⁻
Protons = 35
Br⁻ has gained one electron.
Electrons:
= 35 + 1
= 36
Neutrons:
= 80 - 35
= 45
Answer:
36 electrons and 45 neutrons
(iii) ²⁰¹Hg²⁺
Protons = 80
Hg²⁺ has lost two electrons.
Electrons:
= 80 - 2
= 78
Neutrons:
= 201 - 80
= 121
Answer:
78 electrons and 121 neutrons
THE JOURNEY BEYOND - ACTIVITIES
Activity 1
Q. Design and perform an experiment to show and compare that water always contains hydrogen and oxygen in the same ratio, regardless of its source.
Activity in simple words
Take equal or known masses of purified water collected from different sources, such as:
- tap water,
- river water,
- borewell water,
- rainwater.
First purify the samples. The water can then be decomposed into hydrogen and oxygen under proper laboratory conditions.
Answer / Conclusion
Pure water from every source contains hydrogen and oxygen in the same fixed proportion.
By mass:
Hydrogen : Oxygen = 1 : 8
This supports the Law of Constant Proportions.
Activity 2
Q. Compare atoms and ions of any three elements. Show the number of electrons before and after ion formation using bar graphs.
Activity in simple words
Choose three elements and compare the number of electrons in their neutral atoms and their ions.
Answer: Example data
| Element | Electrons in atom | Ion formed | Electrons in ion |
|---|---|---|---|
| Sodium | 11 | Na⁺ | 10 |
| Magnesium | 12 | Mg²⁺ | 10 |
| Chlorine | 17 | Cl⁻ | 18 |
These values can be shown using bar graphs.
Activity 3
Q. Make a card game with cations and anions.
Activity in simple words
Prepare cards showing ions such as:
- Na⁺
- K⁺
- Mg²⁺
- Ca²⁺
- Al³⁺
- Cl⁻
- O²⁻
- SO₄²⁻
- NO₃⁻
- CO₃²⁻
Players have to match positive and negative ions so that the total charge becomes zero.
Examples
Na⁺ + Cl⁻ → NaCl
Mg²⁺ + 2Cl⁻ → MgCl₂
2Al³⁺ + 3O²⁻ → Al₂O₃
2K⁺ + SO₄²⁻ → K₂SO₄
This game helps students practise writing chemical formulae.
THE QUEST CONTINUES
Q. Are there any chemical changes that do not obey the Law of Conservation of Mass?
Answer:
No. In a chemical change, mass is conserved.
Sometimes it may appear that mass has decreased because a gas escapes from an open container. It may also appear to increase when a substance combines with a gas from the air.
But when all reactants and all products are included, the total mass before and after the chemical reaction remains the same.