Long Answer Questions - 5 Marks

Q. 1. Arrange the following in order of decreasing masses:
(i) 10²³ molecules of CO₂ gas             (ii) 0.1 g atom of silver
(iii) 1 gram of carbon                        (iv) 0.1 mole of H₂SO₄
(v) 10²³ atoms of calcium.
(Given Atomic masses: Ag = 108 u, S = 32 u, N = 14 u, Ca = 40 u)

Ans. (i) 1 mole of CO₂ = 44 g = 6.02 x 10²³ molecules        

               i.e.,             6.02 x 10²³ molecules of CO₂ = 44 g of CO₂           

             

         (ii) 1 g atoms of Ag       = Gram atomic mass of Ag = 108 g

                ∴  0.1 g atom of Ag   = 0.1 × 108 g = 10.8 g

         (iii) 1 g  of carbon        = 1g

         (iv) 1 mole of H₂SO₄ = Gram molecular mass
                                            = 2 x 1+ 32 + 4 x 16 = 98 g
                ∴  0.I mole of H₂SO₄ = 0.1 x 98 g = 9.8 g

         (v) 1 mole of Ca = 40 g = 6.02 × 10  ²³ atoms of Ca

              i.e., 6.02 x 10²³ atoms of Ca have mass = 40 g

              

                                                                       = 6.64 g
Thus, masses in the decreasing order are:
0.1 g atom of Ag > 0.1 mole of H₂SO₄ > 10²³ molecules of CO₂ > 10²³ atoms of Ca > I g of carbon

Q.2. Calculate the number of aluminium ions (Al³+) in 0.056 g of alumina (Al₂O₃).

Ans. Molecular mass of alumina (Al₂0₃) = 2 × Al³⁺ + 3 × 0^2-

                                                                        = 2 × 27 u + 3 × 16u

                                                                        = 102 u

                                Gram molecular mass = 102 g

                            I mol of alumina (Al₂0₃) = 102 g

                                             102 g of Al₂O₃ = 1 mol

     ∴                                  

                                                                     = 5.49 x 10^-4 mol

We know that one mol of alumina contains 2 mol of Al³⁺ ions.

      ∴     5.49 x 10^-4 mol of Al₂O₃ contains 2 x 5.49 x 10^-4 mol of Al³⁺ ions

      ∴                  Number of Al³⁺ ions in 0.056 g = 2 × 5.49 × 10^-4 x 6.022 x 10²³

                                                                               = 6.613 x 10²⁰ ions of AI³⁺

Q. 3. Calculate the mass per cent of each element present in the molecule of calcium carbonate.

Ans. Molecular formula of calcium carbonate = CaCO₃

         Molecular mass of CaCO₃ = 1 × Ca + 1 x C + 3 × 0

                                                        = 1 x 40 u + 1 x 12 u + 3 x 16 u = 100 u

         Gram molecular mass = 100 g/mol

         1 mol of CaCO₃ = 100 g

         

                                                        

         (b) Mass % of carbon in CaCO₃ =  
 

                                                                 

         (c) Mass % of oxygen in CaCO₃   

                                                                

Q. 4. 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 of magnesium elements have a mole ratio of 3 : 5.

Ans. (a) CO₂ has molar mass = 44 g mol^-1

               5 moles of Co₂ have molar mass = 44 x 5 = 220 g

               H₂0 has molar mass = 18 g mol^-1

               5 moles of H₂O have mass = 18 × 5 g = 90 g

        (b) Number of moles in 240 g Ca metal =  

              Number of moles in 240 g of Mg metal = 

                   Ratio is 6 : 10

                   Or,         3 : 5

Q.5. Find the ratio of mass of the combining elements in the following compounds:
        (a) CaCO₃                                                    (b) MgCl₂                  (c) H₂SO₄
        (d) C₂H₅OH             (e) NH₃                                          (f) Ca(OH)₂

Ans. (a) CaCO₃                                    (b) MgCl₂                 (c) H₂SO₄
             Ca : C : O × 3                               Mg : Cl × 2                H × 2 : S : O ×  4
             40 : 12 : 16 x 3                             24 : 35.5 × 2              1 × 2 : 32 : 16 × 4
             40 : 12 : 48                                   24 : 71                       2 : 32 : 64
             10 : 3 : 12                                                                       1 : 16 : 32

        (d) C₂H₅OH                                     (e) NH₃                      (f) Ca(OH)₂
              C × 2 : H × 6 : O                             N : H × 3                        Ca : O × 2 : H × 2
             12 × 2 : 1 × 6 : 16                             14 : 1 × 3                       40 : 16 × 2 : 1 × 2
             24 : 6 : 16                                         14 : 3                             40 : 32 : 2
             12 : 3 : 8                                                                                 20 : 16 : 1

Q. 6. Calcium chloride when dissolved in water dissociates into its ions according to the following equation.
        CaCl₂ (aq) → Ca²⁺ (aq) + 2Cl^- (aq)
Calculate the number of ions obtained from CaCl₂ when 222 g of it is dissolved in water.

Ans. I mole of calcium chloride = 111 g
         ∴     222 g of CaCl₂ is equivalent to 2 moles of CaCl₂
        Since 1 formula unit CaCl₂ gives 3 ions, therefore, 1 mole of CaCl₂ will give 3 moles of ions.
        2 moles of CaCl₂ would give 3 x 2 = 6 moles of ions.
                 Number of ions        = Number of moles of ions x Avogadro number
                                                     = 6 × 6.022 × 10²³
                                                     = 36.132 x 10²³
                                                     = 3.6132 × 10²⁴ ions.

Q. 7. What is a mole? What is the unit of mole? How many molecules are there in a certain mass of a substance?

Ans. A mole is the amount of a substance which contains the same number of chemical units (atoms,molecules or ions) as there are atoms in exactly 12 g of carbon-12. The unit of mole is given by the symbol 'mol'.

We know that Avogadro number is 6.022 × 10²³
Number of molecules in a certain mass
                                              

                                              

where 'W' is the mass of the substance in which number of molecules is to be calculated and 'M' is the molecular mass of the substance.

Q. 8. The difference in the mass of 100 moles each of sodium atoms and sodium ions is 5.48002 g. Compute the mass of an electron.

Ans. A sodium atom and ion differ by one electron. For 100 moles each of sodium atoms and ions there would be a difference of 100 moles of electrons.

Mass of 100 moles of electrons = 5.48002 g

Mass of 1 mole of electron = 

Mass of one electron = 

                                      = 9.1 × 10^-28 g

                                 = 9.1 × 10^-31 kg

Q.9. The mass of one steel screw is 4.11g. Find the mass of one mole of these steel screws. Compare this value with the mass of the Earth (5.98 × 10²⁴kg). which one of the two is heavier and by how many times?

Ans.  1 mole of steel screws = 6.022 x 10²³ screws

                                Mass of 1 screw = 4.11g

                        ∴     Mass of 1 mole of screws = 4.11 x 6.022 × 10²³ g

                                = 24.75 x 10²³ g

                               = 2.475 x 10²⁴ g

One mole of screw weighs = 2.475 x 10²⁴ g = 2.475 x 10²¹ kg

                       

Mass of Earth is 2.4 × 10³ times the mass of screws.

The Earth is 2400 times heavier than one mole of screws.

 

Q.10. Compute the number of ions present in 5.85 g of sodium chloride.

Ans. 

         or 0.1 moles of NaCl particle.

        Each NaCI particle is equivalent to 2 ions, i.e., one Na⁺ and one Cl^-

      ⇒ Total moles of ions = 0.1 × 2 = 0.2 moles

        Number of ions       = 0.2 × 6.022 × 10²³

         = 1.2042 x 10²³ ions

Q. 11. A gold sample contains 90% of gold and the rest copper. How many atoms of gold are present in one gram of this sample of gold?

Ans. One gram of gold sample will contain 

         Number of moles of gold =

                                                       = 

        One mole of gold contains N_A atoms = 6.022 x 10²³

         ∴     0.0046 mole of gold will contain = 0.0046 × 6.022 × 10²³

              = 2.77 x 10²¹ atoms

Q. 12. Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is 9.1 x 10^-28 g). Which one is heavier?

Ans. Mass of 1 mole of aluminium atom = Molar mass of aluminium = 27 g mol^-1.
         An aluminium atom needs to lose three electrons to become an ion, Al³⁺.
         For one mole of Al³⁺ ion, three moles of electrons are to be lost.
               The mass of three moles of electrons = 3 x (9.1 x 10^-28) x 6.022 x 10²³ g
                                                                                  = 27.3 x 6.022 x 10^-5g
                                                                                  = 164.400 x 10^-5 g = 0.00164 g
                                               Molar mass of Al³⁺ = (27- 0.00164) g mol^-1
                                                                                  = 26.9984 g mol^-1
                                                              Difference = 27- 26.9984
                                                                                  = 0.0016 g

Q. 13. A silver ornament of mass ‘m’ gram is polished with gold equivalent to 1% of the mass of silver. Compute the ratio of the number of atoms of gold and silver in the ornament.

Ans. Mass of silver = m g

         

        Number of atoms of silver = 

        Number of atoms of gold = 

         Ratio of number of atoms of gold to silver = Au : Ag

                                                                                     

                                                                                    = 108 : 100 x 197
                                                                                    = 108 : 19700
                                                                                    = 1 : 182.41

Q.14. A sample of ethane (C₂H₆) gas has the same mass as 1.5 x 10²⁰ molecules of methane (CH₄). How many C₂H₆ molecules does the sample of gas contain?

Ans. Mass of 1 molecule of CH₄ =  

         Mass of 1.5 x 10²⁰ molecules of methane = 

         Mass of 1 molecule of C₂H₆ =

        Mass of molecules of C₂H₆ =

         ∴                   Number of molecules of ethane = 

 

 

Q. 15. In photosynthesis, 6 molecules of carbon dioxide combine with an equal number of water molecules through a complex series of reactions to give a molecule of glucose having a molecular formula C₆H₁₂O₆. How many grams of water would be required to produce 18 g of glucose? Compute the volume of water so consumed assuming the density of water to be 1g cm^-3.

Ans. 

         

         1 mole of glucose needs 6 moles of water

         180 g of glucose needs (6 × 1 8) g of water

         Ig of glucose will need  of water.

         18 g of glucose would need  of water = 10.8 g

          Volume of water used =

                                                  

Q. 16. Calculate the ratio between the mass of one atom of hydrogen and mass of one atom of silver.

Ans. 1 mole of H atoms = 1 g
         1 mole of H atoms = 6.022 × 10²³ atoms.

         Mass of 6.022 x 10²³ atoms of H = 1g

         ∴   Mass of one atom of H = 

                                                       = 1.66 x 10^-24 g

       1 mole of silver atoms = 108 g

       1 mole of silver contains 6.022 × 10²³ atoms

         ∴   6.022 x 10²³ atoms of silver          = 108 g

         ∴   Mass of one atom of silver atom =

                                                                        = 1.793 × 10^-22g

             Ratio between masses of silver and hydrogen atoms

                                                                      

                                                                        = 1.080 × 10²