Teachoo Questions

Class 9
Chapter 3 Class 9 - Atoms And Molecules

## Calculate the molar mass of the following substances.

(a) Ethyne, C 2 H 2

(b) Sulphur molecule, S 8

(c) Phosphorus molecule, P 4 (Atomic mass of phosphorus = 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO 3

Molar mass of C = 12g

Molar mass of H = 1g

Molar mass of S = 32g

Molar mass of P = 31g

Molar mass of Cl = 35.5g

Molar mass of N = 14g

Molar mass of O = 16g

(a) Ethyne, C 2 H 2

Molar mass of C 2 H 2 = 2 x Molar mass of C + 2 x Molar mass of H

= 2x 12g + 2 x 1g

= 24g + 2g

= 26g

(b) Sulphur molecule, S 8

Molar mass of S 8 = 8 x Molar mass of S

= 8x 32g

= 256g

(c) Phosphorus molecule, P 4

Molar mass of P 4 = 4 x Molar mass of P

= 4 x 31g

= 124g

(d) Hydrochloric acid, HCl

Molar mass of HCl = 1 x Molar mass of H + 1 x Molar mass of Cl

= 1x 1g + 1 x 35.5g

= 1g + 35.5g

= 36.5g

(e) Nitric acid, HNO 3

Molar mass of HNO 3 = 1 x Molar mass of H + 1 x Molar mass of N + 3 x Molar mass of O

=1x 1g + 1 x 14g + 3 x 16g

= 1g + 14g + 48g

= 63g

## Find number of moles of H 2 in 10g of H 2

Given mass of H 2 = m =10g

Molar Mass of H 2 = M=  2g

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

= m/M

= 10g / 2g

= 5 moles

## How many moles is 12.044 x 10 23 atoms of He

Number of atoms of the He = N = 12.044 x 10 23

Avogadro's Constant = N O = 6.022 x 10 23

We know that,

Number of moles of a substance = Number of particles of the substance / Avogadro's Constant

= N/N O

= 11.044 x 10 23 / 6.022 x 10 23

=  2 moles of He atoms

## How many moles are there in 52g of Helium?

Atomic Mass of Helium = 4 u

So Molar Mass of  Helium = 4 gram

It means that Mass of 1 mole of Helium = 4 grams

We can also write as

4 grams = 1 mole

1 gram =1/4 mole

52 grams = (¼) x 52 mole

52 grams = 13 moles

So,

n = Given mass / Molar mass

= 52 / 4

= 13 moles

## Calculate the Molar Mass of Calcium Carbonate (CaCO 3 )

1 molecule of CaCO 3 consists of : 1 Calcium (Ca) atom

1 Carbon (C ) atom

3 Oxygen (O) atoms

M of CaCO 3 = (1 x mass of Ca) + (1 x mass of C) + (3 x mass of O)

M of CaCO 3 = (1 x 40) + (1 x 12) + (3 x 16)

M of CaCO 3 = (1 x 40) + (1 x 12) + (3 x 16)

M of CaCO 3 = 100g/mol

## What is the mass of—

(a) 1 mole of nitrogen atoms?

(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?

(c) 10 moles of sodium sulphite (Na 2 SO 3 )?

We know that,

Mass of 1 mole of a substance = Molar mass of that substance

(a) 1 mole of nitrogen atoms

Molar mass of Nitrogen (N) = M =14g

Therefore, mass of 1 mole Nitrogen atoms = Molar mass of nitrogen = 14g

(b) 4 moles of aluminium atoms

Molar Mass of Aluminium = 27g

Mass of 1 mole Aluminium atoms = Molar mass of aluminium = 27g

Mass of 4 moles Aluminium atoms = 4 x  Molar mass of aluminium

= 4 x 27g

= 108g

(c) 10 moles of sodium sulphite (Na 2 SO 3 )

Mass of 1 mole Na 2 SO 3 atoms =  2 x Molar mass of Na + 1 x Molar Mass of S + 3 x Molar Mass of O

= 2 x 23g + 1 x 32g + 3 x 16g

= 46g + 32g + 48g

= 126g

Mass of 10 moles Na 2 SO 3 atoms   = 10 x  Molar mass of Na 2 SO 3

= 10 x 126g

= 1260g

## Convert into mole.

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide.

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

(a) 12 g of oxygen gas

Given mass of O 2 = m = 12g

Molar Mass of O 2 = M = 32g

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

= m/M

= 12g / 32g

= 6 /16  moles

= ⅜ moles

(b) 20 g of water

Given mass of H 2 O = m = 20g

Molar Mass of H 2 O = M = 18g

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

= m/M

= 20g/18g

= 10/9  moles

(c) 22 g of carbon dioxide

Given mass of CO 2 = m = 22g

Molar Mass of CO 2 = M = 44g

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

= m/M

= 22g/44g

= 1/2  moles

= 0.5 moles

## What is the mass of:

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

Therefore,

Mass of the substance = Number of moles x Molar mass of the substance

(a) 0.2 mole of oxygen atoms

Given mass of O = m = ?

Molar Mass of O = M = 16g

Number of moles = n = 0.2 = 2/10

We know that,

Mass of the substance = Number of moles x Molar of the substance

m = n x M

= (2 /10) x 16

= 32 /10

= 3.2g

(b) 0.5 mole of water molecules

Given mass of H 2 O = m = ?

Molar Mass of H 2 O = M = 18g

Number of moles = n = 0.5 = 5/10 = 1/2

We know that,

Mass of the substance = Number of moles x Molar of the substance

m = n x M

= (1/2) x 18

=  18/2

= 9g

## Calculate the number of molecules of sulphur (S 8 ) present in 16 g of solid sulphur.

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

n  = m/M ----- (1)

Also,

Number of moles of a substance = Number of particles of the substance / Avogadro's Constant

n  = N / N O

n  = N / 6.022 x 10 23 ------- (2)

From the above 2 formulae, we can say that,

n = m / M = N / 6.022 x 10 23   ---- (3)

Given mass of sulphur molecule(S 8 ) = m = 16g

Molar mass of Sulphur Molecule (S 8 ) = M = 256g

We need to find Number of molecules, ie, N

Putting values in (3), we get

16 / 256 = N / 6.022 x 10 23

N = 6.022 x 10 23   x (16 / 256)

= 6.022 x 10 23   / 16

= 3.011 x 10 23 /8

= 0.37 x 10 23 molecules of Sulphur(S 8 )

= 3.7 x 10 22 molecules of Sulphur (S 8 )

## 3.42 g of sucrose are dissolved in 18 g of water in a beaker. The number of oxygen atoms in the solution are

(a) 6.68 x10 23

(b) 6.09 x10 22

(c) 6.022 x 10 23

(d) 6.022.x 10 21

(a) 6.68 x10 23

Explanation:

Number of moles = Given mass of substance / Molecular mass of substance

n Sucrose   = m / M

n Sucrose   = 3.42 g / 342g

n Sucrose   = 0.01 mole

1 mol of sucrose → 11 x N 0 atoms of O

0.01 mol of sucrose → 0.01 x 11 x N 0 atoms of O

→ 0.11 x N 0 atoms of O

n H20   = m / M

n H20   = 18 g / 18g

n H20   = 1 mole

1 mol of water → N 0 atoms of O

Total number of O atoms = (0.11 + 1) x N 0

=  1.11 x N 0

= 1.11 x 6.022 x 10 23

= 6.68 x 10 23

## If one mole of carbon atoms weighs 12 grams, what is the mass (in grams) of 1 atom of carbon?

Given,

Mass of 1 mole of Carbon  = Molar mass of C = 12g

1 mole of carbon = 6.0 22 x 10 23 atoms of Carbon

Therefore,

Mass of 6.0 22 x 10 23 atoms of Carbon = 12 g

Mass of 1 atom of Carbon = 12 / 6.0 22 x 10 23

≈ 2 x 10 - 23 g

## If 1.4g of Calcium Oxide is formed by the complete decomposition of Calcium Carbonate, then the amount of Calcium Carbonate taken and the amount of Carbon Dioxide formed will be respectively?

1. 2.2g and 1.1g
2. 1.1g and 2.5g
3. 2.5g and 1.1g
4. 5.0 and 1.1g

1. c) 2.5g and 1.1g

Calcium Carbonate → Calcium Oxide + Carbon Dioxide

CaCO 3   → CaO + CO 2

X → 1.4g + Y

Molecular mass of CaO = 56g

Number of moles of CaO = 1.4/56

= 0.025

Molecular mass of CaCO 3 = 100g

Molecular mass of CO 2 = 44g

So, 0.025 moles of CaCO 3  =  0.025 x 100g

= 2.5g of CaCO 3

And, 0.025 moles of CO 2  =  0.025 x 44g

= 1.1g of CO 2

## Which of the following correctly represents 360 g of water?

(i) 2 moles of H 2 O

(ii) 20 moles of water

(iii) 6.022 x 10 23 molecules of water

(iv) 1.2044 x10 25 molecules of water

(a) (i)

(b) (i) and (iv)

(c) (ii) and (iii)

(d) (ii) and (iv)

(d) (ii) and (iv)

Explanation:

(i) 2 moles of H 2 O → 1 mole of H 2 O = 18g → 2 moles = 36g ≠ 360g

(ii) 20 moles of water → 1 mole of H 2 O = 18g → 20 moles = 360g

(iii) 6.022 x 10 23 molecules of water → 1 mole of water molecules = 18g ≠ 360g

(iv) 1.2044 x10 25 molecules of water → 20 moles of water = 360g

## Which of the following pairs have the same number of atoms?

A - 16 g of O 2 (g) and 4 g of H 2 (g)

B - 16 g of O 2 and 44 g of H 2

C - 28 g of N 2 and 32 g of O 2

D - 12 g of C(s) and 23 g of Na(s)

C - 28 g of N 2 and 32 g of O 2

D - 12 g of C(s) and 23 g of Na(s)

Explanation:

One mole of any species (atoms,molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.

1 mole of an item = 6.022 x 10 23 items

In 28 g of N 2 and 32 g of O 2

28 g of N 2 = 1 mole of N 2 = 2 x 6.022 x 10 23 atoms

32 g of O 2 = 1 mole of O 2 = 2 x 6.022 x 10 23 atoms

In 12 g of C(s) and 23 g of Na(s)

12 g of C(s) = 1 mole of C = 6.022 x 10 23 atoms

23 g of Na(s) = 1 mole of C = 6.022 x 10 23 atoms

## Which of the following contains the greatest number of atoms?

A - 1g of butane

B - 1g of Nitrogen

C - 1g of silver

D - 1g of water

A - 1g of butane

Explanation

Molecular mass of Butane (C 4 H 10 ) is 58g/mol

There are 14 atoms in the molecule

So, 1g of butane means 1/58 moles = 0.0172 moles

Total number of atoms = 14 x 0.0172 x 6.022 x 10 23 = 1.45 x 10 23 atoms

Molecular mass of Nitrogen (N 2 ) is 28g/mol

There are 2 atoms in the molecule

So, 1g of nitrogen means 1/28 moles = 0.0357 moles

Total number of atoms = 2 x 0.0357 x 6.022 x 10 23 = 0.43 x 10 23 atoms

Molecular mass of Silver (Ag) is 108g/mol

There are is 1 atom in the molecule

So, 1g of silver means 1/108 moles = 0.0092 moles

Total number of atoms = 0.0092 x 6.022 x 10 23 = 0.054 x 10 23 atoms

Molecular mass of Water (H 2 O) is 18g/mol

There are 3 atoms in the molecule

So, 1g of water means 1/18 moles = 0.055 moles

Total number of atoms = 3 x 0.055 x 6.022 x 10 23 = 1.003 x 10 23 atoms

Thus 1g of butane has greatest number of atoms

## Which sample contains the largest number of atoms?

A - 1mg of C 4 H 10

B - 1mg of N 2

C - 1mg of Na

D - 1ml of H 2 O

D - 1ml of H 2 O

Explanation:

1mg of C 4 H 10 (14 atoms in 1 molecule) = (14 x N x 10 -3 ) / 58 = 2.41 x 10 -4 x N atoms

1mg of N 2 (2 atoms in 1 molecule) = (2N x 10 -3 ) / 28 = 7.14 x 10 -5 x N atoms

1mg of Na (1 atom in 1 molecule) = (N x 10 -3 ) / 23 = 4.34 x 10 -5 x N atoms

1ml of H 2 O (3 atoms in 1 molecule) = 1g of H 2 O = 3N / 18 = 2.41 x 10 -4 x N atoms

Thus 1ml of H 2 O has largest number of atoms

## How many moles of Magnesium Phosphate (Mg 3 (PO 4 ) 2 ) will contain 0.25 mole of oxygen atoms?

A - 3.125 x 10 -2

B - 1.25  x 10 -2

C - 2.5 x 10 -2

D - 2 x 10 -2

A - 3.125 x 10 -2

Explanation:

In one molecule of Mg 3 (PO 4 ) 2 there are 8 Oxygen atoms

So in 1 mole of Mg 3 (PO 4 ) 2 molecules , there will be 8 moles of Oxygen atoms.

1 mole of Mg 3 (PO 4 ) 2 → 8 moles of O atoms

X                → 0.25 moles of O atoms

Cross multiplying,

0.25 x 1 = 8X

X = 0.25/8

X = 3.125 x 10 -2

3.125 x 10 -2 moles of Magnesium Phosphate will contain 0.25 mole of oxygen atoms.

## Which has the maximum number of molecules?

A - 7g N 2

B - 2g H 2

C - 16g NO 2

D - 16g O 2

B - 2g H 2

Explanation:

Number of moles = Given mass of substance / Molecular mass of substance

n  = m / M

7g N 2 → 7 / 28 = 0.25 moles of molecules

2g H 2 → 2 / 2 = 1 mole of molecules

16g NO 2 → 16 / 46 = 0.347 moles of molecules

16g O 2 → 16 / 32 = 0.5 moles of molecules

Thus 2g H 2 has the most number of molecules.

## How many gram atoms of H and S are contained in 0.40 mole of H 2 S?

1 mole of a substance = 1 gram atom

In this case

H 2 S has 2 H atoms and 1 S atom

1 mole of H 2 S → 2 gram atoms of H and 1 gram atom of S

So, 0.4 moles of H 2 S →

2 x 0.4 = 0.8 gram atoms of H

1 x 0.4 = 0.4 gram atoms of S

## Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?

To find which one has more atoms, we’ll find the number of moles of both.

Since 1 mole of any substance has the same number of atoms, the element having more number of moles will have more atoms .

Let,

Given mass of Na = m n =100g

Given mass of Fe = m f =100g

We know that,

Molar mass of a substance = Atomic mass of the substance (in g)

Therefore,

Molar mass of Na = M n = 23g

Molar mass of Fe = M f = 56g

Now,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

For Na, = m n /M n

= 100/23

≈  4.34 moles

For Fe, = m f /M f

= 100/56

≈  1.78 moles

Since, Number of moles of Na > Number of moles of Fe .

100g of Na will have more atoms than 100g of Fe .

## Calculate the number of moles and molecules in 22g of Acetic Acid.

Acetic Acid has the formula CH 3 COOH

The molecular mass is thus → (12x2 + 1x4 + 16x2)

= 60g

Number of moles n = Given mass / Molar mass

n = 22/60

n = 0.3666 moles

1 mole                →        6.022 x 10 23 molecules (according to mole concept)

0.3666 moles     →       X molecules

X = 0.3666 x 6.022 x 10 23

X = 2.207 x 10 23 molecules

So, 22g of Acetic acid has 0.3666 moles and 2.207 x 10 23 molecules

## 24 g of Carbon reacts with some Oxygen to make 88g of Carbon Dioxide. Find out the amount of Oxygen used.

The reaction for formation of Carbon Dioxide is;

C + O 2 → CO 2

1 mole of C reacts with 1 mole of O 2 to give 1 mole of CO 2

12g of C reacts with 1g of O 2 to give 44g of CO 2

It is given that 24g of C gives 88g of CO 2

Which is double the amount of C and CO 2

So even O 2 will be double the amount → 32g

## In 2 moles of acetaldehyde (CH 3 CHO) calculate the following

1. Number of moles of C
2. Number of moles of H
3. Number of moles of O
4. Number of molecules of CH 3 CHO

1. 1 mole of acetaldehyde has 2 C atoms ( C H 3 C HO )

So, 1 mole of acetaldehyde has 2 moles of C atoms

That means, 2 mole of acetaldehyde has 4 moles of C atoms

1. 1 mole of acetaldehyde has 4 H atoms ( C H 3 C H O )

So, 1 mole of acetaldehyde has 4 moles of C atoms

That means, 2 mole of acetaldehyde has 8 moles of C atoms

1. 1 mole of acetaldehyde has 1 O atom ( CH 3 CH O )

So, 1 mole of acetaldehyde has 1 moles of O atom

That means, 2 mole of acetaldehyde has 2 moles of C atoms

1. 1 mole     →    6.022 x 10 23 molecules (according to mole concept)

2 moles   →   X molecules

X = 2 x 6.022 x 10 23

X = 12.044 x 10 23 molecules

## Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)

We know that,

Number of moles of a substance = Given mass of the substance / Molar mass of the substance

n  = m/M ----- (1)

Also,

Number of moles of a substance = Number of particles of the substance / Avogadro's Constant

n  = N / N O

n  = N / 6.022 x 10 23 ------- (2)

From the above 2 formulae, we can say that,

n = m / M = N / 6.022 x 10 23   ---- (3)

Given mass of Aluminium ion (Al 2 O 3 )= m = 0.051g

Molar mass of Aluminium Oxide (Al 2 O 3 ) = M = 102g

We need to find Number of ions, ie, N

Putting values in (3), we get

0.051 / 102 = N / 6.022 x 10 23

N = 6.022 x 10 23   x (0.051 / 102)

= 6.022 x 10 23   x (51 x 10 -3 ) / 102

= 3.011 x 10 (23-3)

= 3.011 x 10 20   molecules

1 molecule of Al 2 O 3 has 2 Al 3+ ions

So, number of Al 3+ ions in 0.051 molecules of Al 2 O 3 is;

3.011 x 10 20 x 2 = 6.022 x 10 20

Thus, 6.022 x 10 20 Al 3+ ions are present in 0.051 molecules of Al 2 O 3

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