Class 9 · Science · 2026-27

The mole concept and atomic structure with Answers

A mole is a counting unit, exactly like a dozen. A dozen is twelve of anything; a mole is 6.022 × 10²³ of anything - atoms, molecules or ions. Almost all the difficulty in this chapter comes from being handed three formulas before that one sentence, so it is worth reading again before starting.

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The mole concept and atomic structure

Class 9 Science · Chemistry · 29 questions with answers

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A mole is a counting unit, exactly like a dozen. A dozen is twelve of anything; a mole is 6.022 × 10²³ of anything - atoms, molecules or ions. Almost all the difficulty in this chapter comes from being handed three formulas before that one sentence, so it is worth reading again before starting.

Calculates moles from mass and mass from moles
Converts between moles and number of particles
Distinguishes molecules from atoms when counting
Writes electronic configuration and deduces valency
Distinguishes isotopes from isobars

Exercise 1 of 3 · 11 questions

Calculate. Show the molar mass you used.

Write the answer · Warm-up

Here’s one done for you

Moles = given mass ÷ molar mass, so 22 g of CO₂ is 22 ÷ 44 = 0.5 mol. Write the molar mass as its own line before dividing - that is where the marks are and where the errors are. If your child cannot say what a mole counts, stop and say it: a mole is 6.022 × 10²³ particles, the way a dozen is twelve. The formulas make sense afterwards and not before.

1)How many moles are there in 44 g of carbon dioxide?
2)How many moles are there in 22 g of carbon dioxide?
3)How many moles are there in 9 g of water?
4)What is the mass of 2 moles of oxygen gas, O₂?
5)How many moles are there in 117 g of sodium chloride?
6)How many moles are there in 36 g of water?
7)How many moles are there in 80 g of sodium hydroxide?
8)How many moles are there in 56 g of nitrogen gas?
9)What is the mass of 0.5 mole of calcium carbonate?
10)How many moles are there in 34 g of ammonia?
11)What is the mass of 0.5 mole of sulphuric acid?

Answers

1) Molar mass of CO₂ = 12 + 32 = 44 g/mol. So 44 g is exactly 1 mole.
2) 22 ÷ 44 = 0.5 mol.
3) Molar mass of H₂O = 18 g/mol. So 9 ÷ 18 = 0.5 mol.
4) Molar mass of O₂ = 32 g/mol. So 2 × 32 = 64 g.
5) Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol. So 117 ÷ 58.5 = 2 mol.
6) Molar mass of H₂O = 18 g/mol. 36/18 = 2 moles.
7) Molar mass of NaOH = 40 g/mol. 80/40 = 2 moles.
8) Molar mass of N₂ = 28 g/mol. 56/28 = 2 moles.
9) Molar mass of CaCO₃ = 100 g/mol. 0.5 × 100 = 50 g.
10) Molar mass of NH₃ = 17 g/mol. 34/17 = 2 moles.
11) Molar mass of H₂SO₄ = 98 g/mol. 0.5 × 98 = 49 g.

Exercise 2 of 3 · 8 questions

Calculate the number of particles, or the mass. Watch whether the question asks for atoms or molecules.

Write the answer · Guided

Here’s one done for you

One mole of O₂ contains 6.022 × 10²³ molecules but 1.2044 × 10²⁴ atoms, because each molecule holds two atoms. This is the single most common error in the chapter, and the fix is a habit rather than knowledge: underline the word 'atoms' or 'molecules' in the question before starting. The arithmetic is identical; only that word decides whether to multiply by two.

1)How many molecules are there in 0.5 mol of carbon dioxide?
2)How many atoms are there in 1 mole of oxygen gas, O₂?
3)How many moles are there in 12 g of carbon?
4)What is the mass of 3.011 × 10²³ molecules of water?
5)How many molecules are there in 0.25 mole of a gas?
6)What is the mass of 0.5 mole of carbon dioxide?
7)How many atoms are there in 0.5 mole of sodium?
8)How many molecules are there in 36 g of water?

Answers

1) 0.5 × 6.022 × 10²³ = 3.011 × 10²³ molecules.
2) 1 mole of O₂ contains 6.022 × 10²³ molecules, and each molecule has 2 atoms - so 1.2044 × 10²⁴ atoms.
3) Atomic mass of carbon is 12, so 12 ÷ 12 = 1 mol.
4) 3.011 × 10²³ is half of 6.022 × 10²³, so 0.5 mol. Mass = 0.5 × 18 = 9 g.
5) 0.25 × 6.022 × 10²³ = 1.5055 × 10²³ molecules.
6) Molar mass of CO₂ = 44 g/mol, so 0.5 × 44 = 22 g.
7) 0.5 × 6.022 × 10²³ = 3.011 × 10²³ atoms.
8) 36 g is 2 moles, so 2 × 6.022 × 10²³ = 1.2044 × 10²⁴ molecules.

Exercise 3 of 3 · 10 questions

Give the number of protons, neutrons and electrons, the electronic configuration, and the valency.

Write the answer · Practice

Here’s one done for you

For sodium, neutrons = mass number - atomic number = 23 - 11 = 12. The configuration 2, 8, 1 then gives the valency directly: with one electron in the outermost shell, sodium loses it, so the valency is 1. Chlorine's 2, 8, 7 needs one more to fill the shell, so its valency is also 1 - by gaining rather than losing. Valency is never something to memorise; it falls out of the configuration every time.

1)Sodium: atomic number 11, mass number 23
2)Chlorine: atomic number 17, mass number 35
3)Magnesium: atomic number 12, mass number 24
4)Oxygen: atomic number 8, mass number 16
5)Are ¹⁷Cl-35 and ¹⁷Cl-37 isotopes or isobars? What about ²⁰Ca-40 and ¹⁸Ar-40?
6)Nitrogen: atomic number 7, mass number 14.
7)Calcium: atomic number 20, mass number 40.
8)Aluminium: atomic number 13, mass number 27.
9)What is the difference between an isotope and an isobar?
10)Why is the mass of an atom concentrated in its nucleus?

Answers

1) 11 protons, 11 electrons, 23 - 11 = 12 neutrons. Configuration 2, 8, 1. Valency 1, since it loses one electron.
2) 17 protons, 17 electrons, 35 - 17 = 18 neutrons. Configuration 2, 8, 7. Valency 1, since it gains one electron.
3) 12 protons, 12 electrons, 24 - 12 = 12 neutrons. Configuration 2, 8, 2. Valency 2.
4) 8 protons, 8 electrons, 16 - 8 = 8 neutrons. Configuration 2, 6. Valency 2.
5) Cl-35 and Cl-37 are isotopes - same atomic number, different mass number. Ca-40 and Ar-40 are isobars - same mass number, different atomic number.
6) 7 protons, 7 neutrons, 7 electrons.
7) 20 protons, 20 neutrons, 20 electrons.
8) 13 protons, 14 neutrons, 13 electrons.
9) Isotopes have the same atomic number but different mass numbers; isobars have the same mass number but different atomic numbers.
10) Because protons and neutrons both sit in the nucleus and each is about 1,836 times heavier than an electron, so the electrons contribute almost nothing to the mass.

While your child works

Ask what a mole counts before any formula. If your child cannot answer, that is the whole problem, and it takes two minutes to fix.
Underline 'atoms' or 'molecules' in every question. That word alone decides whether to multiply by the number of atoms per molecule.
Valency comes from the configuration. If it is being memorised, the chapter has not landed.

Before you print it

What is on this worksheet

3 exercises, 29 questions, about 35 minutes. Every exercise has an answer key, printed on its own page so it can be kept back. 3 exercises start with a question already worked out, so you can check the method before helping.

Exercise	What the child does	Questions	Worked example
1	Written answers	11	Yes
2	Written answers	8	Yes
3	Written answers	10	Yes

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About this worksheet

What is the mole concept, simply?

A mole is a counting unit, like a dozen. A dozen is twelve of anything; a mole is 6.022 × 10²³ of anything - atoms, molecules or ions. Most of the difficulty comes from meeting the formulas before that sentence.

Why is 1 mole of O₂ sometimes 6.022 × 10²³ and sometimes twice that?

Because it contains 6.022 × 10²³ molecules and each molecule holds two atoms, giving 1.2044 × 10²⁴ atoms. Underlining 'atoms' or 'molecules' in the question before starting removes the error entirely.

Does this Class 9 Science worksheet come with answers?

Yes. Every exercise has a full answer key, and it prints on its own page at the end rather than beside the questions - so you can hand your child the sheet and keep the answers back. Some exercises also show one question already worked out, with the method rather than just the result, which is the part most answer keys leave out.

Is there a free printable PDF?

Yes, and there is no sign-up, no email and no account. The download button at the top of the sheet gives you the whole thing - all 29 questions across 3 exercises, plus the answer key - laid out to print on A4.

How long does this worksheet take?

About 35 minutes for 29 questions, though that is a guide rather than a target. It is designed to be split: the exercises are ordered so stopping halfway still leaves a complete piece of work, which matters more than finishing at this age.

Can I use it if my child is in a different class?

Often, yes - these topics are taught across several years and schools vary in when they arrive. Judge by the questions rather than the class on the label. If the first exercise is a struggle the level below is the better start, and the same topic at other levels is linked further up this page.

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