## Topic outline

• ### Chapter 3 - Atoms And Molecules – 5

• Atoms And Molecules

Q81. Convert into mole.

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide.

Ans. (a) Atomic mass of oxygen (O) = 16u

1 mole of O2 = Molecular mass of O2 in grams

= 2 x 16 g = 32 g

So, the mass of 1 mole of oxygen gas is 32 g

Now, 32 g of oxygen gas = 1 mole

Therefore, 12 g of oxygen gas = 1/32 x 12

= 0.375 mole

Thus, 12 grams of oxygen gas are equal to 0.375 mole.

(b) Atomic mass of hydrogen (H) = 1u

Atomic mass of oxygen (O) = 16 u

1 mole of H2O = Molecular mass of H2O in grams

= 2 x Mass of ‘H’ + Mass ‘O’

= (2 x 1) + 16

= 18 g

So, the mass of 1 mole of water is 18 g

Now, 18 g of water = 1 mole

Therefore, 20 g of water = 1/18 x 20

= 1.11 mole

Thus, 20 grams of water are equal to 1.11 mole.

(c) Atomic mass of carbon (C) = 12u

Atomic mass of oxygen (O) = 16 u

1 mole of CO2 = Molecular mass of CO2 in grams

= Mass of ‘C’ + 2 x Mass ‘O’

= 12 + (2 x 16)

= 44 g

So, the mass of 1 mole of carbon dioxide is 44 g

Now, 44 g of carbon dioxide = 1 mole

Therefore, 22 g of carbon dioxide = 1/44 x 22

= 1/2

= 0.5 mole

Thus, 22 grams of carbon dioxide are equal to 0.5 mole.

Q82. 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)?

Ans. To calculate the number of atoms in 100 grams of sodium:

23g of Na contains = 6.022 x 1023

1g of Na contains = 6.022 x 1023/23

100g of Na contains = 6.022 x 1023 x 100/23

= 2.618 x 1024 atoms

To calculate the number of atoms in 100 grams of iron:

56g of Fe contains = 6.022 x 1023

1g of Fe contains = 6.022 x 1023/56

100g of Fe contains = 6.022 x 1023 x 100/56

= 1.075 x 1024 atoms

Q83. 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

Ans. (d) Atoms aggregate in large numbers to form the matter that we can see, feel or touch. This is statement is not true.

Explanation:

Atoms aggregate in large numbers to form the matter. But we cannot see the matter with naked eyes.

Q84. Which of the following would weigh the highest?

(a) 0.2 mole of sucrose (C12 H22 O11)

(b) 2 moles of CO2

(c) 2 moles of CaCO3

(d) 10 moles of H2O

Ans. (c) 2 moles of CaCO3 would weigh the highest.

Explanation:

(a)Mass of 1 mole of sucrose (12 x12) + (1 x22) + (16×11)= 342g

Mass of 0.2 mole of sucrose = 342 x 0.2 = 68.4g

(b)Mass of 1 mole of CO2 = 12 + (16 x 2) = 44g

Mass of 2 moles of CO2 = 44 x 2 = 88g

(c)Mass of 1 mole of CaCO3 = 40 + 12 + (16 x 3) = 100g

Mass of 2 moles of CaCO3 = 100 x 2 = 200g

(d)Mass of 1 mole of H20 = 2 + 16 = 18g

Mass of 10 moles of H20 = 18 x 10 = 180g

Q85. Which of the following has maximum number of atoms?

(a) 18g of H2O

(b) 18g of O2

(c) 18g of CO2

(d) 18g of CH4

Ans. (d) 18g of CH4

Explanation:

Number of atoms

= (mass substance x no. of atoms x 6.022 x 1023)/molar mass

(a) Number of atoms in 18g of H2O

= (18 x 6.022 x 1023 x 3)/18

= 18.066 x 1023

= 1.8066 x 1024

(b) Number of atoms in 18g of O2

= (18 x 6.022 x 1023 x 2)/32

= 6.774 x 1023

(c) Number of atoms in 18g of CO2

= (18 x 6.022 x 1023 x 3)/44

= 7.390 x 1023

(d) Number of atoms in 18g of CH4

= (18 x 6.022 x 1023 x 5)/16

= 3.387 x 1024

Q86. Which of the following contains maximum number of molecules?

(a) 1g CO2

(b) 1g N2

(c) 1g H2

(d) 1g CH4

Ans. (c) 1g H2

Explanation:

(a) Molar mass of CO2 = 44g

Number of molecules in 44g of CO2 = 6.022 x 1023

Number of molecules in 1g of CO2 = (6.022 x 1023)/44

= 1.37 x 1022

(b) Molar mass of N2 = 28g

Number of molecules in 28g of N2 = 6.022 x 1023

Number of molecules in 1g of N2 = (6.022 x 1023)/28

= 2.15 x 1022

(c) Molar mass of H2 = 2g

Number of molecules in 2g of H2 = 6.022 x 1023

Number of molecules in 1g of H2 = (6.022 x 1023)/2

= 3.011 x 1023

(d) Molar mass of CH4 = 16g

Number of molecules in 16g of CH4 = 6.022 x 1023

Number of molecules in 1g of CH4 = (6.022 x 1023)/16

= 3.76 x 1022

Q87. 3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of oxygen atoms in the solution are

(a) 6.68 × 1023

(b) 6.09 × 1022

(c) 6.022 × 1023

(d) 6.022 × 1021

Ans. (a) 6.68 × 1023

Explanation:

Molar mass of sucrose C12H22C11 = 12x12+1x22+16x11 =342g

342g = 1 mole of sucrose

3.42g = 0.01 mole of sucrose

1 mole of sucrose contains, O atoms = 11 x 6.022 x 1023

0.01 mole of sucrose contains, O atoms = 0.01 x 11 x 6.022 x 1023

= 6.6242 x 1022

18g of H2O = 1 mole of water

1 mole of water contains, O atoms = 6.022 x 1023

Adding number of O atoms present in 3.42g of sucrose and 18g of water, we get, (6.6242 x 1022 + 6.022 x 1023)

= 1022 (6.6242 + 60.22)

= 66.844 x 1022

= 6.68 x 1023

Q88. Verify by calculating that

(a) 5 moles of CO2 and 5 moles of H2O do not have the same mass.

(b) 240 g of calcium and 240 g magnesium elements have a mole ratio of 3:5.

Ans. (a) Molar mass of CO2 = 12 + 2 x 16 = 44g

1 mole of CO2 has a mass = 44g

5 moles of CO2 have a mass = 44 x 5 = 220g

Molar mass of H2O = 2 x 1 + 16 = 18g

1 mole of H2O has a mass = 18g

5 mole of H2O have a mass = 5 x 18g = 90g

Thus, verified that 5 moles of CO2 and 5 moles of H2O do not have the same mass.

(b) Molar mass of calcium = 40g

40 g of calcium = 1 mole

240 g of calcium = 1/40 x 240 = 6 mol

Molar mass of magnesium = 24 g

24 g of magnesium = 1 mole

240 g of magnesium = 1/24 x 240 = 10 mol

Ratio = 6:10 = 3:5

Q89. Find the ratio by mass of the combining elements in the following compounds.

(a) CaCO3 (d) C2H5OH (b) MgCl2 (e) NH3 (c) H2SO4 (f) Ca(OH)2

Ans. (a) CaCO

=Ca(1) : C(1) : O(3)

= 40:12:16x3

= 40:12:48

= 10:3:12

(b) MgCl

=Mg(1) : Cl(2)

= 24:35.5x2

= 24:71

(c) H2SO4

=H(2) : S(1) : O(4)

= 2x1:32:16x4

= 2:32:64

= 1:16:32

(d) C2H5OH

=C(2) : H(6) : O(1)

= 12x2:1x6:16x1

= 24:6:16

= 12:3:8

(e) NH3

=N(1) : H(3)

= 14:1x3

= 14:3

(f) Ca(OH)2

=Ca(1) : O(2) : H(2)

= 40:16x2:1x2

= 40:32:2

= 20:16:1