**Q1: Define mole**

Answer: A mole (or mol) is defined as the amount of substance which contains equal number of particles (atoms / molecules / ions) as there are atoms in exactly 12.000g of carbon-12.

One mole of carbon-12 atom has a mass of exactly 12.000 grams and contains 6.02 × 10

^{23}atoms.

A mol is just a number like a dozen. A dozen equals to 12 eggs , a gross of Pencil equals to 144 Pencil. Similarly, mol is equal to 6.022 × 10

^{23}(Avagadro constant). Mol is also known as chemist dozen.

The value 6.022 × 10

^{23}is known as Avogadro Constant (N

_{A}), after the Italian scientist who first recognized the importance of the mass/number relationship

**Q2: Why was there need to use this number called mole?**

Answer: Atoms and molecules are extremely small in size and their numbers in even a small amount

of any substance is really very large. To handle such large numbers, a unit of similar magnitude is required.

In SI system, mole (symbol, mol) was introduced as seventh base quantity for the amount of a substance.

**Q3: 1 mol of chlorine atom contains**

(a) 6.022 × 10

^{23}atoms

(b) one atom

(c) 35.5 g of Cl

(d) All of the above.

Answer: Both (a) and (b) are correct. 1 mol of Cl atom = 36.5 g of Cl (molar mass)

= 6.02 × 10

^{23}atoms.

**Q4: 1.0 mole of Chlorine molecule (Cl**

_{2}) contains**(i) how many number of molecules.**

**(ii) how many number of atoms.**

**(iii) how much it weighs.**

Answer: (i) 1.0 mol of Cl

_{2}contains 6.022 × 10

^{23}molecules.

(ii) One molecule of Cl

_{2}contains 2 toms of Cl. ∴ Cl

_{2}contains 2 × 6.022 × 10

^{23}atoms

i.e. 12.44 × 10

^{23}atoms

(iii) Molar mass of Cl is 35.5 gm/mol. 1.0 mol of Cl

_{2}weighs = 2 × 35.5 = 71.1 g

*Mole in terms of number,*

**1 Mole of particle =**

**6.022 × 10**

^{23}particles**Q5: Which of the following is correct option?**

**1.0 mole of NH**

_{3}(ammonia) contains ...(a) 6.022 × 10

^{23}molecules

(b) 4 mol of atoms

(c) 1 mol of Nitrogen atoms

(d) 3 × 6.022 × 10

^{23}of H atoms

Answer: All of the above options are correct.

**Q6: What is atomic mass unit (amu)?**

Answer: Atomic mass unit is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom. One atomic mass unit also called one Dalton.

Mass of one mole of C-12 atoms = 12 g = mass of 6.022 × 10

^{23}C-12 atoms

1 amu = 1g per mol = 1/N

_{A}= 1/ (6.022 × 10

^{23}) = 1.66 × 10

^{-24}g.

**Q7: What is the mass of one**

^{12}C**atom? Express it in grams as well as in amu.**

Answer: one mole of C-12 atoms = 12 g = mass of 6.022 × 10

^{23}C-12 atoms

∴ mass of 1 C-12 atom = 12g ÷ 6.022 × 10

^{23}= 1.994 × 10

^{-23}g

or = 12g ÷ 6.022 × 10

^{23}= 12g × 1.66 × 10

^{-24}g = 12 amu.

^{}

**Q8: What is molar mass?**

Answer: The mass of one mole of an element or one mole of compound is referred as molar mass.

It is expressed as g mol

^{-1}.

Example:

molar mass of Mg = 24 g mol

^{-1}.

molar mass of methane (CH

_{4}) = (12 + 4) g mol

^{-1}= 16 g mol

^{-1}.

**Q9: What is Gram atomic mass or molar mass of an element?**

Answer: Gram atomic mass or molar mass of an element is mass of 1 mol of atoms or atomic mass expressed in grams. For example, atomic mass of Mg = 24u, therefore, molar mass of Mg is 24 grams per mol. Molar mass of an element is also called

**one gram atom**.

**Q10: What is Gram molecular mass or molar mass of molecular substance?**

Answer: Gram molecular mass or molar mass of a molecular substance is the mass of 1 mol of molecules or molecular mass expressed is grams. For example, molecular mass of H

_{2}O is 18u (2u + 16u), therefore, molar mass of H

_{2}O is 18 g mol

^{-1}.

*Mole in terms of mass,*

**No. of Moles = Mass ÷ Molar mass**

**Q11: In 14g of N**

_{2}(nitrogen gas), calculate**(i) number of moles (take molar mass of 28 g mol**

^{-1})**(ii) number of molecules**

**(iii) number of atoms**

Answer:

(i) No. of moles = 14g ÷ 28 g mol

^{-1}= 0.5 moles

(ii) No. of molecules = (number of moles) × N

_{A}= 0.5 × 6.022 × 10

^{23}= 3.011 × 10

^{23}molecules

(iii) One molecule of N

_{2}contains 2 N atoms.

No. of atoms = 2 × 3.011 × 10

^{23}= 6.022 × 10

^{23}atoms = 1 N

_{A}atoms.

**Q12: One gram molecule H**

_{2}O contains how many moles of molecules and number of molecules?Answer: 1 gram molecule H

_{2}O = 1 mol of molecule H

_{2}O = 6.022 × 10

^{23}molecules of H

_{2}O

**Q13: How many moles of molecule are present in 15g of C**

_{2}H_{4}(ethylene). Given atomic mass of C = 12.0 amu and of H = 1.0 amu.Answer: Molecular mass of C

_{2}H

_{4}= 2 × 12.0 amu + 4 × 1.0 amu = 24 + 4 = 28amu

∴ Molar mass of C

_{2}H

_{4}= 28 g mol

^{-1}

No. of moles of molecule C

_{2}H

_{4}= mass ÷molar mass = 15 ÷ 28 =

**0.536 mol.**

**Q14: Calculate number of bromide ions in 3 moles of mercury(II) bromide.**

Answer: Mercury(II) bromide (HgBr

_{2}) in ionic form in an aqueous solution is

HgBr

_{2}→ Hg

^{+2}+ 2Br

^{-1}

∴ 1 molecule of HgBr

_{2 }contains 2 Br

^{-1 }ions.

⇒ 1 mol of HgBr

_{2 }contains 2 mol Br

^{-1 }ions.

∴ 3 molecule of HgBr

_{2 }contains (2×3) = 6 Br

^{-1 }ions. = 6 × 6.022 × 10

^{23}= 1.807 × 10

^{24 }ions

**Q15: What is Molar Volume?**

Answer: One mole of any gas contains 22.4 liters at N.T.P or S.T.P. (0°C and 1atm/760 mm of Hg), the volume is called molar volume of standard molar volume. For molecular gases (e.g. CH

_{4}), it is expressed as gram molecular volume and for atomic gases (e.g. He gas), it is expressed as gram atomic volume.

e.g. One mol of CH

_{4}= 22.4 litres at STP gram molecular volume = 1 gram molecule of CH

_{4}

= 6.022 × 10

^{23}molecules of CH

_{4}= 16g of CH

_{4}

*Mole in terms of molar volume, at STP*

**No. of Moles =**

**Volume of gas (dm**

^{3}) ÷ 22.4**(dm**

^{3}**mol**

^{-1}**)**

**Q16: What is the volume in litres at S.T.P. of 3 moles of hydrogen sulfide H**

_{2}**S gas?**

Answer: At S.T.P., 1 mole = 22.4 litres molar volume

3 moles = 3 × 22.4 = 67.21 litres of Hydrogen Sulphide (H

_{2}S).

**Q17: A balloon is filled with He gas at N.T.P. The volume of the balloon is 2.24 dm**

^{3}, find the amount of He gas in terms of moles is present.Answer: At N.T.P., 22.4 dm

^{3}contains 1 mol of Helium gas.

2.24 dm

^{3}will have = 2.24 × 1/ 22.4 = 0.1 mol.

**Q18: Calculate the molar mass of glucouse (C**

_{6}H_{12}O_{6}_{}).Answer:

Atomic mass of C = 12u

Atomic mass of H = 1 u

Atomic mass of O = 16 u

No. of C atoms glucouse = 6

No. of H atoms glucouse = 12

No. of O atoms glucouse = 6

Molar mass of glucouse = 6 × 12 + 12 × 1 + 6 × 16 = 180g

⇒ 1 mole or 6.022 × 10

^{23}molecules of glucouse (C

_{6}H

_{12}O

_{6}) weighs =

**180g**

**Q19: A 12.0 g sample of gas occupies 19.2 L at STP. What is the molecular weight of this gas?**

Answer: According to Avagadro's law, at n.t.p. 1 molar volume of gas = 22.4 L

At n.t.p. 19.2 L gas weighs = 12.0 g

At n.t.p. 22.4 L gas weighs = 12.0 x 22.4 /19.2 =

**14.0g / mol**

**Q20(NCERT): Calculate the molecular mass of the following:**

(i) H

_{2}O

(ii) CO

_{2}

(iii) CH

_{4}

Answer:

Molecular mass is the sum of atomic masses of the elements present in a molecule. It is obtained by multiplying the atomic mass of each element by the number of its atoms and adding them together.

(i) Molecular mass of Water (H

_{2}O) = (2× Atomic mass of H) + (1 × Atomic mass of Oxygen)

= 2 × 1.008u + 1 × 16.00u = 2.016 u + 16.00 u = = 18.016 u =

**18.02 u**

(ii) Molecular mass of CO

_{2}= (1 × Atomic mass of C ) + (2 × Atomic mass of O)

= 1 × 12.011u + 2 × 16.00u = = 12.011 u + 32.00 u =

**44.01 u**

(iii) Molecular mass of CH

_{4}= (1 × Atomic mass of C ) + (4 × Atomic mass of H)

= 1 × 12.011u + 4 × 1.008u = = 12.011 u + 4.032 u =

**16.043 u**

**Q21(NCERT): How much copper can be obtained from 100 g of copper sulphate (CuSO4)?**

Answer: 1 mole of CuSO

_{4}contains = 1 mole of Cu + 1 mol of S + 4 moles of O.

Molar mass of CuSO

_{4}= (63.5g) + (32.00g) + 4(16.00g)

= 63.5 + 32.00 + 64.00 = 159.5 g

Since, 1 mole of CuSO4 contains 1 mole of Copper.

⇒ 159.5 g of CuSO4 contains 63.5 g of copper.

100g of CuSO4 will have Cu = (63.5 × 100) / 159.5 =

**39.81g**

**Q22(NCERT):**

**What will be the mass of one**

^{12}C atom in g?Answer: 1 mole of carbon atoms = 6.022 × 10

^{23}atoms of carbon

= 12 g of carbon

Mass of one

^{12}C atom = 12 ÷ 6.022 × 10

^{23}= 1.994 × 10

^{-23}g

**Q23 (NCERT): Calculate the number of atoms in each of the following**

**(i) 52 moles of Ar**

(ii) 52 u of He

(ii) 52 u of He

**(iii) 52 g of He.**

Answer:

(i) 1 mole of Ar = 6.022 × 10

^{23}atoms of Ar

52 mol of Ar = 52 × 6.022 × 10

^{23 }atoms of Ar

=

**3.131 × 10**atoms of Ar

^{25 }(ii) 1 atom of He = 4 u of He

⇒ 4 u of He = 1 atom of He

1 u of He atom of He = 1/4 atoms of He

52u of He atom of He = 52 × 1/4 =

**13 atoms of He**

(iii) 4 g of He = 6.022 × 10

^{23}atoms of He

52 g of He atoms of He = 6.022 × 10

^{23}× 52 / 4 =

**7.8286 × 10**atoms of He

^{24}(In progress...)

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