Questions - Answers
 

1. What is Stoichiometry?

A: The quantitative relationship existing between the reactants and products in a

chemical reaction.

2. What is disproportionation reaction?

A: The reaction in which same element undergoes oxidation as well as reduction.

       e.g.: 3 Cl₂ + 6 NaOH → 5 NaCl + NaClO₃ + 3 H₂O

3. Write the electrode reactions in Daniel cell

A: At anode: Zn  ⇌ Zn⁺² + 2 e^- (Oxidation)

          At cathode: Cu⁺² + 2e^- ⇌ Cu (Reduction)

         Cell reaction: Zn + Cu⁺² (aq.)  ⇌ Zn⁺² (aq.)  +   Cu (redox reaction)

4. Calculate the oxidation numbers of (a) Mn in MnO₄^- (b) Cr in K₂Cr₂O₇

A: (a) x - 8 = -1        ∴  x  =  -1 + 8  =  +7

          (b) 2(+1) + 2x + 7(-2) = 0    2x - 12 = 0  x = +6

5. Give the equivalent weight of (a) H₃PO₃          (b) Na₂CO₃

6. What weight of O₂ is obtained when 0.245 g of KClO₃ decomposes completely into KCl and O₂?

A: 2 KClO₃  

 2 KCl + 3 O₂

          2 × 122.5 g -------------- 3 × 32 g

          ∴  0.245 g ----------------- ?

7. What volume of CO₂ is obtained at STP by heating 10 g of CaCO₃?

A: CaCO₃    CaO + CO₂

           100 g -------------------- 22.4 lit

             10 g --------------------- ?

8. Calculate the volume of O₂ at STP required to burn 100 ml of acetylene completely.

A:  2 C₂H₂ + 5 O₂   4 CO₂ + 5 H₂O

           2 × 22400 ml ------------ 5 × 22400 ml

          ∴ 100 ml ------------------- ?

9. How many number of moles of glucose are present in 540 grams of glucose?

A: No. of moles of glucose =   = 3

10. The empirical formula of a compound is CH₂O. Its molecular weight is 90. Calculate the molecular formula of the compound.

A:  Molecular weight = 90

Empirical formula (CH₂O) weight = 12 + 1 + 1 + 16 = 30

n =    = 3

∴  Molecular Formula = (E.F.)_n = (CH₂O)₃ = C₃H₆O₃

11. What is redox concept? Give an example.

A:  The reaction in which both the oxidation as well as reduction reaction takes place is called redox reaction.

eg.: S + O₂   →    S   O₂

        0     0         +4  −2

Oxidation state of S is changed from 0 to +4 (Oxidation).

Oxidation state of Oxygen is changed from 0 to −2 (Reduction).

12. What are significant figures? How many significant figures are present in (a) 2.005 (b) 0.200

A: Meaningful digits with certainity are called significant figures.

a) Significant figures are present in 2.005 = Four

b) Significant figures are present in 0.200 = Three

13. Calculate the normality of the resultant solution, if 150 ml of water is added to 50 ml of 0.1 N Na₂CO₃ solution.

A: Before dilution                  After dilution

V₁ = 50 ml                                V₂ = 50 + 150 = 200 ml

N₁ = 0.1 N                                 N₂ = ?

V₁N₁ = V₂N₂            

 

14. Balance the following equation in acid medium by ion electron (half-reaction) method.

          MnO₄^-  +  SO₃^-2→ Mn⁺²  +  SO₄^-2

A:  MnO₄^-   +   SO₃^-2  →  Mn⁺²  +  SO₄^-2

In this reaction, oxidation state of Mn is changed from + 7 (in MnO₄^-) to +2 (in (Mn⁺²). So this is reduction reaction. The remaining half is oxidation reaction.  

Adding L.H.S. of oxidation and reduction halves, adding R.H.S. of oxidation and reduction halves and bringing similar terms together.

 5 SO₃^-2 + 5 H₂O + 2 MnO₄^- + 16 H⁺ + 10 e^-→ 5 SO₄^-2 + 10 H⁺  +  2 Mn⁺² + 8 H₂O + 10 e^-

2 MnO₄^-  +  5 SO₃^-2  +  6 H⁺→ 2 Mn⁺²  +  5 SO₄^-2  +  3 H₂O

15. Balance the following equation in alkaline (basic) medium by ion electron method.

                                  Cr(OH)₃ + IO₃^-→ CrO₄^-2 + I^-

A: In this reaction, oxidation state of I is changed from + 5 (in IO₃^-) to -1 (in I^-). So This is reduction.

Adding L.H.S. of oxidation and reduction haves adding R.H.S. of oxidation and reduction halves and bringing similar terms together

 2 Cr(OH)₃  +  10 OH^-  +  IO₃^-  +  3 H₂O  +  6 e^-→ 2 CrO₄^-2  +  8 H₂O + 6 e^- +  I^-  +  6 OH^-                          2 Cr(OH)₃ + IO₃^- + 4 OH^-→ 2 CrO₄^-2 + I^- + 5 H₂O
 

16. In an organic compound on analysis was found to contain 40% Carbon, 53.3% Oxygen and 6.6% Hydrogen. The vapour density is 90. Find its molecular formula.

A: Check whether the sum of percentages of all the elements is equal to 100 or not. If not, the remaining element is Oxygen (if not mentioned any element).

1. Divide all the elements by their atomic weights

                 C         H          O

2.    3.3     6.6     3.3

      Divide with lowest no. 3.3

                  1           2          1

∴ E.F.  =  C H₂O       E.F. Wt. = 12 + 2 + 16  =  30

Molecular Weight = 2 × Vapour density

                                = 2 × 90 = 180        

        
                           

                      ∴ M.F. = (E.F) × n

                                 = (CH₂O) × 6

              Molecular formula = C₆H₁₂O₆ 

17. Explain the different types of redox reactions.

A:  Combination reaction: The reaction in which two or more atoms or molecules (one is in elemental form) combine to form a new molecule.

eg.: S + O₂→  SO₂

Decomposition reaction: The reaction in which a molecule breaks in two or more products (One is in elemental form)

eg.: 2 H₂O →  2 H₂ + O₂

Displacement Reaction: The reaction in which a less reactive atom (or ion) is displaced by a more reactive atom (or ion).

eg.: CuSO₄ + Zn → ZNSO₄ + Cu

Disproportionation Reaction: The reaction in which same element undergoes oxidation as well as reduction.

eg.: 3 Cl₂ + 6 NaOH →  5 NaCl + NaClO₃ + 3 H₂O

18. Chemical analysis of a Carbon compound gave 10.06% C, 0.84% H and 89.1% Cl by weight. Calculate the empirical formula of the compound

Empirical formula = CHCl₃

19. Calculate the empirical formula of a compound having 26.57% K, 35.36% Cr and 38.07% Oxygen

A:

Empirical formula = K₂Cr₂O₇