Teachoo Questions

Class 9
Chapter 3 Class 9 - Atoms And Molecules

## 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 statements is not true about an atom?

(a) Atoms are not able to exist independently.

(b) Atoms are the basic units from which molecules and ions are formed.

(c) Atoms are always neutral in nature.

(d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch.

(a) Atoms are not able to exist independently.

Explanation:

Atoms of noble gases are capable of existing independently.

They have completely filled outermost shell so they can exist stably

## 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

## You are provided with a fine white coloured powder which is either sugar or salt. How would you identify it without tasting?

Sugar →

1. Charred on heating and turns black
2. Will not conduct electricity when dissolved in water as it is covalent

Salt →

1. Does not get charred on heating
2. Conducts electricity when dissolved in water as it is ionic

## What is the SI prefix for each of the following multiples and submultiples of a unit?

(a) 10 3

(b) 10 -2

(c) 10 -6

(d) 10 -9

(e) 10 -12

(a) 10 3 →  kilo

(b) 10 -2 →  deci

(c) 10 -6 → micro

(d) 10 -9 → nano

(e) 10 -12 → pico

## What do the following denote?

1. N
2. 2N
3. N 2
4. 2N 2

1. N → 1 Nitrogen atom
2. 2N → 2 Nitrogen atoms

(c) N 2 → 1 Nitrogen molecule

(d) 2N 2 → 2 Nitrogen molecules

## 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

## Show how Law of Conservation of Mass can be applied to Physical Changes.

Law of Conservation of mass states that the Total Mass of Reactants is equal to the Total Mass of Products

This is applicable to physical reaction also.

For example:

If 100 grams of ice is melted, it produces 100 grams of water

Ice          →             Water

100 grams 100 grams

Reactant Product

Here, the mass of the initial and final stage of matter is the same.

## Q9

Why is OH in Mg(OH) 2 written within brackets and not in NaOH?

OH - is a polyatomic ion; it is an ion made of Oxygen (O) and Hydrogen (H).

• In NaOH, there is only 1 OH - ion

The valencies interchange while writing formula as shown below

Hence Formula of Sodium hydroxide is NaOH

Since the number of polyatomic ions is 1, brackets are not required. We write it as NaOH and not Na(OH)

• In Mg(OH) 2 there are 2 OH - ions.

These valencies interchange while writing formula as shown below

Hence Formula of Magnesium hydroxide is Mg(OH) 2

Hydroxide is written in brackets as 2 is applicable to O and H which means there are 2 OH - ions (2 O and 2 H atoms)

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