AP 9th Class Physical Science 3rd Lesson Important Questions Atoms and Molecules

These AP 9th Class Physics Important Questions 3rd Lesson Atoms and Molecules will help students prepare well for the exams.

AP 9th Class Physical Science 3rd Lesson Atoms and Molecules Important Questions

Class 9 Physical Science Chapter 3 Important Questions – 2 Marks

Question 1.
How does a chemical formula determine?
Answer:

1. The chemical formula of a molecular compound is determined by the valency of each element.
2. In ionic compounds, the charge on each ion is used to determine the chemical formula of the compound.

Question 2.
What is a relative atomic mass scale ?
Answer:

1. Scientists use the relative atomic mass scale to compare the masses of different . atoms of elements.
2. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom.

Question 3.
Define atomicity. Give examples.
Answer:
The number of atoms contained in a molecule of a substance (element (or) compound) is called its atomicity.

1. Monoatomic – He, Ne
2. Diatomic – H₂, O₂
3. Triatomic – CO₂, NO₂
4. Tetraatomic – P₄
5. Polyatomic – H₂SO₄

Question 4.
What is paramanu according to Maharishi Kanad ?
Answer:

1. An Indian philosopher Maharishi Kanad, postulated that if we go On dividing matter (padarth), we shall get smaller and smaller particles.
2. Ultimately, a stage will come when we shall come across the smallest particles beyond which further division will not be possible. He named these particles Paramanu.

Question 5.
What is an atom according to Democritus ?
Answer:

1. Democritus and Leucippus suggested that a stage Will come when the particles obtained cannot be divided further if we go on dividing matter.
2. Democritus called these indivisible particles atoms (meaning indivisible).

Question 6.
Why should we put a cork on the mouth of the flask while doing the experiment to prove the law of conservation of mass ?
Answer:

1. The cork fixed on the mouth of the flask prevents escaping of any substance from the flask.
2. So, other substances do not enter the flask or escape from the flask.
3. Hence it gives reliable results.

Question 7.
What is valency ? What is its use ?
Answer:

1. The combining power (or capacity) of an element is known as its valency.
2. Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.

Question 8.
What is molecular mass ? What are its units ?
Answer:

1. The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.
2. It is therefore the relative mass of a molecule expressed in atomic mass units (u).

Question 9.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass ?
Answer:
The postulate of Dalton’s atomic theory which is the result of the law of conservation of mass is the first postulate, which states that “all matter is composed of small indivisible particles called atoms, which cannot be created, destroyed or divided.

Question 10.
Define the atomic mass unit.
Answer:

1. The atomic mass unit (amu) is a unit of mass used for expressing the mass of atomic and molecular particles, such as atoms, molecules, ions and isotopes.
2. It is defined as one-twelfth (1/12) of the mass of a neutral atom of carbon-12, which is approximately 1.66 × 10^-27 kilograms

Question 11.
Why is it not possible to see an atom with naked eyes ?
Answer:

1. Atoms are the smallest basic unit of matter that make up everything around us.
2. They are incredibly small, with a typical size of around 0.1 to 0.5 nanometers (1 nanometer = 1 billionth of a metre).
3. This means that atoms are too small to be seen with the naked eye, or even with most microscopes.

Question 12.
In a class, a teacher asked students to write the molecular formula of oxygen. Shamita wrote the formula as O₂ and Priyanka as O. Which one is correct? State the reason.
Answer:
Shamitha’s answer is correct.

Reason:

1. Oxygen is diatomic.
2. Two atoms of oxygen combine to form oxygen molecule.
3. Hence the formula of oxygen molecule will be ‘O₂‘.

Question 13.
Mohith said “H₂ differs from 2H.” Justify.
Answer:
H₂ is the hydrogen molecule in which two hydrogen atoms are combined to form one hydrogen molecule.
2H is the hydrogen atom. Here 2 hydrogen atoms are ready to participate in chemical reaction.

Question 14.
100 g of mercuric oxide decompose to give 92.6 g of mercury and 7.4 g of oxygen. Let us assume that 10 g of oxygen reacts completely with 125 g of mercury to give mercuric oxide. Do these values agree with the law of constant proportions?
Answer:
Proportion of oxygen = 7.4 : 10
Proportion of mercury = 92.6 : 125
⇒ \(\frac{7.4}{10}\) = \(\frac{92.6}{125}\) ⇒ 0.74 = 0.74
∴ They follow law of constant proportions.

Question 15.
Write down the electron distribution of oxygen atom. How many valence electrons does it have ? (Atomic number of oxygen is 8)
Answer:
O(8) = 2, 6
K, L
No.of valence electrons = 6

Question 16.
Mention the postulate of Dalton’s atomic theory which can successfully explain the law of definite proportions.
Answer:
The Law of definite proportions is explained by the following postulate of Dalton. “The relative number and kinds of atoms are constant in a given compound.”

Question 17.
State the Law of constant proportion.
Answer:
Law of constant proportion states that in a chemical compound the elements are always present in a definite proportion by mass.

Question 18.
Write the chemical formula for
a) Zinc phosphate
b) Lead carbonate.
Answer:
a) Zn₃(PO₄)₂
b) PbCO₃

Question 19.
Write the chemical formula of
a) Sodium Carbonate
b) Ammonium Chloride.
Answer:

Question 20.
a) Write the chemical formula of a compound using zinc ion and phosphate ion.
b) Calculate the ratio by mass of atoms present in a molecule of carbon dioxide.
Answer:
a) Zn₃(PO₄)₂.
b) The ratio by mass of constituting elements in carbon dioxide is
CO₂ = 12 : 32
3 : 8

Atoms and Molecules Class 9 Important Questions – 3 Marks

Question 1.
What are ions ? Mention their types and give example.
Answer:
Atoms with some charge are called ions.
Cations : Ions that carry positive charge are called “Cations”. Ex : Na⁺, Mg²⁺.
Anions : Ions that carry negative charge are Called “Anions”. Ex : Cl^–, SO₄^2-.

Question 2.
Explain the law of definite proportions with examples.
Answer:

1. In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8, whatever the source of water.
2. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.
3. Similarly in ammonia, nitrogen and hydrogen are always present in the ratio 14 : 3 by mass, whatever the method or the source from which it is obtained.

Question 3.
What is meant by molecules of an element ?
Answer:

1. The molecules of an element are constituted by the same type of atoms.
2. Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element.
3. A molecule of oxygen consists of two atoms of oxygen, O₂.

Question 4.
Give example of monoatomic, diatomic, tetra atomic and polyatomic elements.
Answer:

Name	Atomicity
Argon	Monoatomic
Helium	Monoatomic
Oxygen	Diatomic
Hydrogen	Diatomic
Nitrogen	Diatomic
Chlorine	Diatomic
Phosphorus	Tetra-atomic
Sulphur	Poly-atomic
Name	Atomicity

Question 5.
Write the chemical formula for hydrogen chloride & hydrogen sulphide by using Criss-cross method.
Answer:
1) Formula of hydrogen chloride Symbol

Formula of the c’dmpound would be HCl.

2) Formula of hydrogen sulphide Symbol

Formula: H₂S

Question 6.
Write the chemical formula for carbon tetrachloride by using Criss – cross method.
Answer:
Formula of carbon tetrachloride

Formula : CCl₄

Question 7.
Write the chemical formula for magnesium chloride by using Criss – cross method.
Answer:
For magnesium chloride, we write the symbol of cation (Mg²⁺) first followed by the symbol of anion (Cl^–). Then their charges are criss-crossed to get the formula.
Formula of magnesium chloride

Formula : MgCl²
Thus, in magnesium chloride, there are two chloride ions (Cl^–) for each magnesium ion (Mg²⁺). The positive and negative charges must balance each other and the overall structure must be neutral.

Question 8.
Lakshmi gives a statement “CO and Co both represent element”. Is it correct ? State reason. .
Answer:
Lakshmi’s statement is incorrect.
Reason :

1. CO stands for carbon monoxide, a compound, which consists carbon and oxygen atoms.
2. This can be identified with the help of both C and O are capital (upper case) letters.
3. Co stands for cobalt, an element.
4. This can be identified with the help of ‘C’ is capital (upper case) letter and ‘o’ small (lower case) letter.

Question 9.
How would you write 2 molecules of Oxygen and 5 molecules of Nitrogen ?
Answer:
2 molecules of oxygen → 2O₂
Reason:

1. Oxygen is diatomic element.
2. Two oxygen atoms combine to form one oxygen molecule.
3. The formula of oxygen molecule is O₂.
   5 molecules of nitrogen → 5N₂

Reason :

1. Nitrogen is also diatomic element.
2. Two nitrogen atoms combine to form one nitrogen molecule.
3. Molecular formula of nitrogen is N₂.

Question 10.
The formula of a metal oxide is MO. Then write the formula of its chloride.
Answer:

1. The valency of oxide is 2 i.e., O^-2.
2. The formula of a metal oxide is given as MO.
3. Hence the valency of the given metal must be 2 i.e., M⁺².
4. Valency of chloride is 1 i.e., Cl^–
5. Therefore according to criss – cross method the formula of given metal chloride will be MCl₂.

Question 11.
Write the chemical formulae of the following.
a) Aluminium nitrate
b) Magnesium hydrogen carbonate
c) Sodium sulphate
Answer:

Question 12.
a) Write down the chemical formula of sodium oxide and magnesium hydroxide.
b) What are polyatomic ions ? Write one example.
c) What is the atomicity of phosphorous and ammonia ?
Answer:
a) Na₂O ; Mg(OH)₂
b) A group of atoms having charge is called polyatomic ion. E.g: CO₃^2-.
c) Atomicity of phosphorus (P) = 4
Atomicity of Ammonia (NH₃) = 4 ⇒ 1 + 3 = 4

Question 13.
Calculate the molar mass of sulphuric acid (H₂SO₄ and glucose (C₆H₁₂O₆).
Answer:
a) Formula of sulphuric acid is H₂SO₄.
Molecular mass of H₂SO₄ = 2 × 1 + 1 × 32 + 4 × 16
= 2 + 32 + 64 = 98 u
∴ Molar mass of H₂SO₄ = 98 g

b) Formula of glucose is C₆H₁₂O₆.
Molecular mass of C₆H₁₂O₆ = (6 × 12) + (12 × 1) + (6 × 16) = 72 + 12 + 96 = 180 u
∴ Molar mass of C₆H₁₂O₆ = 180 g.

Question 14.
The percentage of three elements – calcium, carbon and oxygen in a sample of calcium carbonate is given as: calcium = 40%, carbon = 12.0%, oxygen = 48%.
If the law of constant proportion is true, what weight of these elements will be present in 1.5 gm of another sample of calcium carbonate ?
(Atomic mass of Ca = 40u, C = 12u, O = 16u)
Answer:
Mass of Calcium in 1.5 gm of sample = \(\frac{40}{100}\) × 1.5 = 0.6 gm
Mass of Carbon in 1.5 gm of sample = \(\frac{12}{100}\) × 1.5 = 0.18 gm
Mass of Oxygen in 1.5 gm of sample = \(\frac{48}{100}\) × 1.5 = 0.72 gm

Important Questions on Atoms and Molecules Class 9 – 5 Marks

Question 1.
Distinguish atoms and molecules.
Answer:

Atoms	Molecules
Composition : Atoms are the basic building blocks of matter, consisting of a nucleus of protons and neutrons surrounded by electrons.	Molecules, on the other hand, are composed of two or more atoms chemically bonded together.
Size : Atoms are typically very small, with diameters on the order of angstroms (10-10 metres).	Molecules can vary widely in size depending on the number and types of atoms they contain.
Formation : Adorns can combine with other atoms through chemical bonds to form molecules.	Molecules can also be broken down into their constituent atoms through chemical reactions.

Question 2.
Write the postulates of Dalton’s atomic theory. _
Answer:
According to Dalton’s atomic theory, all matter, whether an element, a compound, or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows :

1. All matter is made of very tiny particles called atoms, which participate in chemical reactions.
2. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
3. Atoms of a given element are identical in mass and chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.

Question 3.
Write the rules to follow while writing a chemical formula.
Answer:
The rules that you have to follow while writing a chemical formula are as follows :
1) The valencies or charges on the ion must balance.

2) When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first.
For example calcium oxide (CaO), sodium chloride (NaCl), iron sulphide (FeS), copper oxide (CuO), etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals and are written on the left.

3) In compounds formed with polyatomic ions, the number of ions present in the compound is indicated by enclosing the formula of the ion in a bracket and writing the number of ions outside the bracket.

For example, Mg (OH)₂. In case the number of polyatomic ions is one, the bracket is not required. For example, NaOH.

Question 4.
Write the chemical formula for (a) aluminium oxide (b) calcium oxide (c) sodium nitrate (d) calcium hydroxide (e) sodium carbonate (f) ammonium sulphate by using Criss – cross method.
Answer:

Question 5.
Formula of calcium hydroxide is Ca(OH)₂ and zinc phosphate is Zn₃(PO₄)₂. Then write the formula to calcium phosphate.
(OR)
Formula of calcium hydroxide is Ca(OH)₂ and zinc phosphate is Zn₃(PO₄)₂. Then write the valencies of calcium and phosphate and then write the formula of calcium phosphate.
Answer:

1. Formula of calcium hydroxide is Ca(OH)₂.
2. From criss-cross method we know that the valency of calcium is 2 i.e., Ca⁺² and hydroxide is 1 i.e., OH^–.
3. Formula of zinc phosphate is Zn₃(PO₄)₂.
4. Valency of Zn is 2 i.e., Zn⁺² and valency of phosphate is 3 i.e., PO₄^-3.
5. Now the formula of calcium phosphate according to criss – cross method is Ca₃(PO₄)₂.

Question 6.
15.9 g of copper sulphate and 10.6 g of sodium carbonate react together to give 14.2 g of sodium sulphate and 12.3 g of copper carbonate. Which law of chemical combination is obeyed ? How ?
Answer:
Reactants :
Mass of copper sulphate = 15.9 g; Mass of sodium carbonate = 10.6 g
Total mass of reactants = 15.9 + 10.6 = 26.5 g

Products:
Mass of sodium sulphate = 14.2 g; Mass of copper carbonate = 12.3 g
Total mass of products = 14.2 + 12.3 = 26.5 g
∴ Total mass of reactants is equal to total mass of products. This is the “Law of conservation of Mass”.

Question 7.
Carbon dioxide is added to 112 g of calcium oxide. The product formed is 200 g of calcium carbonate. Calculate the mass of carbon dioxide used. Which law of chemical combination will govern your answer ?
Answer:

1. Let x g of carbon dioxide is added to 112 g of calcium oxide.
2. The product is 200 g of calcium carbonate.
3. According to law of conservation of mass,
   Total mass of reactants = Total mass of products
   x + 112 = 200 g
   x = 88 g .
   ∴ 88 g of carbon dioxide is used.

Question 8.
Imagine what would happen if we do not have standard symbols for elements.
(OR)
Is it necessary to use symbols for elements ? Write your opinion.
Answer:

1. Chemistry involves a lot of reactions.
2. If we do not have symbols, we have to write their names to represent the reactions.
3. This is very tedious work.
4. To avoid this, we need standard symbols to elements, which are universally accepted.
5. In advanced Studies, balancing of equations, atoms present in a compound, etc. will not be understood without symbols.
6. Simply chemistry will not be developed unless symbols, formulae, etc. are not known.

Question 9.
Write down the formulae of these compounds, using criss – cross method.
i) Sodium oxide
ii) Aluminium chloride
iii) Sodium sulphate
iv) Magnesium hydroxide
Answer:

Question 10.
a) Which postulate of Dalton’s atomic theory is the basis of law of conservation of mass ?
b) Write the name of compounds represented by the following formulae :
i) KNO₃
ii) Al₂(SO₄)₃,
iii) CCl₄,
iv) H₂S
Answer:
a) Atom can neither be created nor destroyed.

b) i) Potassium nitrate
ii) Aluminium sulphate
iii) Carbon tetrachloride
iv) Hydrogen sulphide.

Question 11.
a) What are anions ?
b) Write the chemical formulae of the following compounds.
i) Ammonium hydroxide
ii) Calcium chloride
c) Calculate the molecular mass of H₂SO₄.
Answer:
a) A negatively charged ion (that gains electrons) is called an anion. E.g: Cl^–, Br^–.

b) i) NH₄OH
ii) CaCl₂.

c) Molecular mass of H₂SO₄ = 2 × H + S + 4 × O
= 2 × 1 + 1 × 32 × 4 × 16 = 98 units

Question 12.
a) Write chemical formulae of the following compounds.
i) Sodium phosphate
ii) Ammonium carbonate
b) A sample of vitamin C is known to contain 2.58 × 10²⁴ oxygen atoms. How many moles of oxygen atoms are present in the sample ?
c) Write one word for the following :
i) In a balanced chemical equation, the sum of the masses of reactants and products remains unchanged.
ii) A group of atoms carrying a fixed charge on them.
Answer:
a) i) Na₃(PO₄)
ii) NH₄(HCO₃)

b) Number of moles = \(\frac{\text { Given number of particles }}{\text { Avogadro number}}\)
= \(\frac{2.58 \times 10^{24}}{6.022 \times 10^{23}}\) = 0.4284 × 10 = 4.284 moles

c) i) Law of conservation of mass
ii) Ion.

Question 13.
a) Which point of Dalton’s atomic theory came from law of constant proportions ?
b) State the law of constant proportion. Illustrate with the help of an example.
c) If 18gm of pure water is electrolyzed, 2gm of hydrogen and 16gm of oxygen is obtained. Which law of chemical combination is illustrated by this statement ?
d) Which postulate of Dalton’s atomic theory is the result of law of conservation of mass ?
Answer:
a) Atoms of different elements combine in simple whole number ratios to form chemical compound.
b) A compound prepared by any method contains the same elements in the fixed ratio by mass. For example, H₂O contains hydrogen and oxygen in the ratio 2 : 16 i.e., 1 : 8 by mass.
c) Law of constant proportion.
d) Atoms can neither be sub-divided, created nor destroyed.

Question 14.
When 3 gm of carbon is burnt in 8gm of oxygen, llgm of carbondioxide is produced. What mass of carbondioxide will be formed when 3gm of carbon is burnt in 50gm of oxygen ? Which law of chemical combination will given your answer ? State the law.
Answer:
When 3 gm of carbon is burnt in 8 gm oxygen, llgm of carbondioxide is produced. It means carbon and oxygen are combined in the ratio of 3 : 8 to form carbondioxide. Thus, when there is 3 gm carbon and 50 gm oxygen, then also only 8gm of oxygen will be used and 11 gm of carbondioxide will be formed. The remaining oxygen is not used.

This indicates the law of definite proportions. According to this law, the elements are always present in definite proportion by mass in a chemical substance. All pure samples of a compound contain the same elements combined together in the same proportion by mass. .

Question 15.
a) Write the formula of the compounds formed by the following ions.
i) Cr³⁺ and SO₄^2-
ii) Pb²⁺ and NO₃^1-
b) Which has more number of atoms 100g of sodium or 100g of iron (Na = 23 u, Fe = 56 u) ?
c) State the significance of one mole.
Answer:

b) For Sodium
m = 100g ; M = 23g ; n = \(\frac{\mathrm{m}}{\mathrm{M}}\) = \(\frac{100}{23}\)
N = n × N_o = \(\frac{100}{23}\) × 6.022 × 10²³ = 4.34 × 6.022 × 10²³
For Iron : m = 100g ; M = 56g ; n = \(\frac{\mathrm{m}}{\mathrm{M}}\) = \(\frac{100}{56}\)
N = n × N_o = \(\frac{100}{56}\) × 6.022 × 10²³ = 1.785 × 6.022 × 10²³
Hence 100g sodium has more number of atoms.

C) One mole means 6.022 × 10²³ particles of a substance.

Question 16.
a) Write the chemical formulae of
i) Silver nitrate
ii) Magnesium sulphate
iii) Potassium carbonate
iv) Barium chloride
v) Sodium phosphate
vi) Sulphur dioxide.
b) Define Avogadro Number. Convert 2 moles of magnesium sulphate sample into number of molecules present in the sample.
Answer:
a) i) AgNO₃
ii) MgSO₄
iii) K₂CO₃
iv) BaCl₂
v) Na₂PO₄
vi) SO₂.

b) i) The number of particles present in 1 mole of any substance is fixed, with a value of 6.022 × 10²³. This is an experimentally obtained value. This number is called the Avogadro constant or Avogadro Number.
ii) No. of molecules in 2 moles of MgSO₄ = 2 × 6.022 × 10²³ = 12.044 × 10²³

Question 17.
Two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water. Suggest how the above change is in accordance with the law of conservation of mass (Atomic weight of hydrogen = 1, oxygen = 16)
Solution:
2H₂ + O₂ → 2H₂O
Weight of the reactants taking part in the reaction = 2(2 × 1) + 2 × 16 = 4 + 32 = 36 gm
Weight of the product formed in the reaction = 2[(2 × 1) + 16] = 2(2 + 16) = 36 gm
∴ Weight of the reactants = Weight of the products in the reaction
Thus, the above reaction obeys the law of conservation of mass.

Question 18.
The molecular formula of a compound can be expressed as C_nH_{2n + 2}. If the molecular weight of the compound is 54, find the empirical formula of the compound.
Solution:
Molecular weight of C_nH_{2n + 2} = 12 × n + 1 × (2n + 2)
Molecular weight = 58
12n + 2n + 2 = 58
14n = 56 ⇒ n = \(\frac{56}{14}\) = 4
H_{2n + 2} = H₁₀
Substituting the value of n in C_nH_{2n + 2}, we get molecular formula of the compound as C₄H₁₀.
Therefore, the empirical formula of the compound or the simplest ratio between the molecules is C₂H₅.

Extra Questions on Atoms and Molecules Class 9 – 4 Marks

Question 1.
Fill the table with their symbols and Latin, German or Greek names.
(OR)
Why do we write the symbols of iron, sodium, potassium, gold and copper as Fe, Na, K, Au and Cu respectively ?

Element	Iron	Sodium		Gold	Copper
Latin, German or Greek names		natrium	kalium		Cuprum
Symbol	Fe		K	Au

Answer:

Element	Iron	Sodium	Potasshftri	Gold	Copper
Latin, German or Greek narries	ferrum	natrium	kalium	Arum	Cuprum
Symbol	Fe	Na	K	Au	Cu

Question 2.
Complete the following table.

Anions → ↓ Cations	Chloride	Hydroxide	Nitrate	Sulphate	Carbonate	Phosphate
Sodium	NaCl
Magnesium				MgSO4
Calcium
Aluminium
Ammonium						(NH4)3PO4

Answer:

Question 3.
Read the following information and answer.
Molecular mass of a molecule is the sum of the atomic masses of individual atoms in it.

Element	Atomic number	Atomic mass
Sodium	11	23
Oxygen	8	16
Hydrogen	1	1
Carbon	6	12

i) Find the molecular mass of Na₂CO₃.
Answer:
Molecular mass of Na₂CO₃ = (23 × 2) + 12 + (16 × 3) = 46 + 12 + 48 = 106

ii) If the molecular weight of a compound is 44. Which is made with carbon and oxygen. What its molecular formula ?
Answer:
Molecular weight of a compound = 44
At. mass of Carbon = 12

Remaining at. mass = 44 – 12 = 32 = 16 × 2
Hence, 16 × 2 ⇒ (Oxygen atomic mass × 2) ⇒ O₂
Molecular formula = CO₂

iii) What is the unit of atomic mass ?
Answer:
Unit Of atomic mass = amu
atomic mass unit (amu) is defined as precisely \(\frac{1}{12}\) the mass of an atom of carbon – 12.

iv) On the basis of molecular weights of NaOH and H₂O, which is heavier ?
Answer:
Molecular weight of NaOH = 23 + 16 + 1 = 40
Molecular weight of H₂O = (1 × 2) + 16 = 2 + 16 = 18
Hence, NaOH is heavier than H₂O.

Question 4.
Complete the table.

Anion → ↓ Cation	Chloride	Sulphate	Phosphate
Sodium		Na2SO4
Calcium	CaCl2
Aluminium			AlPO4
Ammonium			(NH4)3PO4

Answer:

Anion → ↓ Cation	Chloride	Sulphate	Phosphate
Sodium	NaCl	Na2SO4	Na3PO4
Calcium	CaCl2	CaSO4	Ca3(PO4)2
Aluminium	AlCl3	Al2(SO4)3	AlPO4
Ammonium	NH4Cl	(NH4)2SO4	(NH4)3PO4