NCERT solution for class 9th // Science // Chapter 3 - Atoms and molecules

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NCERT solution for class 9th // Science  // Chapter 3 - Atoms and molecules

Page number 32. 

Q1 . In a reaction 5.3 g of sodium carbonate reacted with 6 gram of ethanoic acid the products were 2.2 gram of carbon dioxide 0.9 gram of water and 8.2 gram of sodium ethanoate show that these observations are  in agreement with the law of conservation of mass sodium carbonate + ethanoic acid  ----- sodium ethanoate + carbon dioxide + water 

Ans- . Sodium carbonate + Ethanoic acid_____

 Sodium Ethanoate + Carbon dioxide + water 

  5.3 g.     +.  6g ____ 8.2 +   2.2 g + 0.9 g 

11.3g.     ___ 11.3 g 

This shows that during the chemical reaction

The mass of reactant =  The mass of product 


Q2. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water what mass of Oxygen gas would be required to react completely with 3 gram of hydrogen gas 

Ans - Hydrogen : Oxygen_____ H2O

          1:8        =    3: x 

        X = 8x3 /1 

       X = 24 g 

24 gram of the Oxygen gas would be required to react completely with 3 gram of hydrogen gas


Q3. Which postulate of Dalton's atomic theory is the result of law of conservation of mass 

Ans- According to the Dalton atomic theory that atoms are indivisible particles which neither be created nor be destroyed in the chemical reaction

Q4. Which postulate of Dalton atomic theory can explain the law of definite proportion.

Ans- According to the Dalton theory the relative number of the atoms and the kind of the atoms are constant in a given compound

Page number 35

Q1. Define the atomic mass unit

Ans- Unit of mass is used to express the atomic and molecular weight the relative atomic of all elements have been found with respect to atoms of carbon 12 so it's equal to the one twelfth  1/ 12 of the mass of an atom of carbon 12 

Q2. Why is it not possible to see an atom with naked eyes.

Ans- The size of the atoms are very so small the radius of the atom is order is about 10 -10

 m atoms of most elements are not exist independently in atmosphere

Page number 39

Q1. Write down the formula of

i) Sodium oxide 

ii) Aluminium chloride 

iii) Sodium sulphide 

iv) Magnesium hydroxide.

Ans  i) Sodium oxide     -  Na2O 

ii) Aluminium chloride      -AlCl3 

iii) Sodium sulphide      - Na2S 

iv) Magnesium hydroxide   -     Mg(OH) 2                      

Q2. Write down the names of compounds represented by the following formula

  • Al2 (SO4)3
  • CaCl2
  • K 2 SO4
  • KNO3
  • CaCO3

Ans-     Al2 (SO4)3   - Aluminium sulphate 

CaCl2   - Calcium chloride

K 2 SO4  - Potassium sulphate 

KNO3  -   potassium nitrate 

CaCO3  - Calcium carbonate             

Q3. What is mean by the term chemical formula

Ans- Chemical formula is the symbolic representation and its compositions of a compound which gives the number of the kinds of the atom and they are  chemical United in given compound 

Eg - The chemical formula of potassium hydroxide is KOH 

Q4. How many atoms are present in a H2S molecule and PO43-

Ans- H2S    - In H2S - 3 atoms are present two atoms of hydrogen and one atom of sulphur

     PO43-  - In PO43-  five atoms are present one atom of phosphorus and four atom of Oxygen

                    Page number 40 

Q1. Calculate the molecular masses of 

  • H2
  • O2 
  • Cl 2
  •  CO2
  • CH4
  •  C2 H6 
  • C2 H4
  •  NH3 
  • CH3OH.

Ans- 

  • H2   = 1+1 = 2u 
  • O2  = 16+16 = 32u 
  • Cl 2  = 35.5+ 35.5 = 71u
  •  CO2  =  12+ 32 = 44u 
  • CH4  = 12+ 4 = 16u
  •  C2 H6  = 12x2 + 1x6 = 30 u 
  • C2 H4  = 12x2 + 1x4 = 28u 
  •  NH3  = 1x14 + 1x3 = 17u
  • CH3OH  = 1x12+ 1x3+ 1x16 + 1x1 = 32u

Q2. Calculate the formula unit masses of ZnO, Na2O , K2CO3 

given atomic masses of Zn = 65 u,  Na= 23 u ,K =39 u C =12 and O = 16 

Ans -

  •  ZnO = 65u + 16 u = 81 u 
  • Na2O = (23u x 2)+ 16u= 62u
  • K2CO3 = (39u x2 )+ 12u + (16u x3 ) = 138 u

                    Page number 42 

Q1 If one mole of carbon atom weight 12 gram what is the mass of one atom of carbon.

Ans  - 1 mole of carbon atoms = 6.022 x1023  

Weight of one carbon  = 12g 

one atom of carbon = 12 x  6.022 x1023   = 1.99 x 10-23

Q2. Which has more number of atoms 100 grams of sodium or 100 grams of iron atomic mass of a Na = 23u  ,Fe= 56u 

Ans -

a) 23g of Na = 6.022 x 10 2

  100g of Na = 6.022 x 10 23  x 100 / 23

    = 2.617 x 10 24 

b) 56g of Fe = 6.022 x 10 23

100 g of Fe  = 6.022 x 10 23  x 100/56

                    = 1.075 x 10 24

                        Exercise 

Q1. 0.24 g sample of compound of Oxygen and boron was found by analysis to contain 0.096 gram of boron and 0.144 gram of oxygen calculate the percentage of composition of the compound by the weight;?

Ans - Percentage of boron in the sample = 

0.096/ 0.024 x 100 = 40% 

Percentage of oxygen in the sample = 

  0.144/0.24 x 100 = 60%

The sample of compound contains 40%  boron and 60% oxygen by weight

Q 2. When 3.0 g of carbon is burnt in 8.00 gram oxygen carbon dioxide is produced what mass of the carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 gram of oxygen which law of chemical combination will govern your answer?

Ans -.   When 3.0 g of carbon is burnt in 8.00 gram of oxygen then 11.00 gram of carbon dioxide is produced it means that all of the carbon and oxygen are used and carbon and oxygen are combine in the ratio of 3 AS 288 to form the carbon dioxide when there is 3 gram of carbon and 50.0 gram of oxygen combine then only 8 gram of Oxygen will be used and 11.0 gram of carbon dioxide will be formed and the remaining oxygen is not used up this shows that the law of definite proportion which show that in all  compounds the elements are combined In The definite proportion of mass 

Q3 Write any polyatomic ions give examples

Ans - When same or different kind of ions which combine more than 1 atoms and behave as a single unit are called polyatomic ion

eg. - Carbonate ion(CO3 2- )

  a compounds ions which made up of two types of the atom join together that is the carbon and oxygen

Q4. Write the chemical formula of following

Magnesium chloride

 Calcium oxide 

Copper nitrate 

Aluminium chloride 

Calcium carbonate

Ans-  

Magnesium chloride     -MgCl2

 Calcium oxide -        Cao

Copper nitrate -    Cu(NO3)2

Aluminium chloride  -    AlCl3

Calcium carbonate     - CaCO3

Q5. Give the names of the elements present in the following compound

  • Quicklime
  •  Hydrogen bromide
  •  Baking powder 
  • Potassium sulphate

Ans-     Quicklime  -   Calcium and oxygen

 Hydrogen bromide   -Hydrogen and bromine.

 Baking powder   -Sodium hydrogen carbon and oxygen

Potassium sulphate  -Potassium Sulphur and oxygen                                       

                                                     

Q6. Calculate the molar mass of the following substances

Ethyne- C2 H2 

Sulphur molecule- S8

 Phosphorus molecule-P4 

Hydrochloric acid-HCl

 Nitric acid - HNO3 

.

Ans- 

Ethyne C2 H2  = (2 x 12) + ( 2x 1) = 26 g

Sulphur molecule - S8 = 8 x 32= 256g 

 Phosphorus molecule P4   = 4. x  31 = 124g 

Hydrochloric acid HCl = (1 X1) + ( 1 x35.5) = 36.5 

 Nitric acid  HNO3 . = (1 X1) + ( 1 x 14) + ( 3 x 16 ) = 63 g 

Q7. What is the mass of

a)1 mole of nitrogen atoms 

b)4 moles of aluminium atoms 

10 moles of sodium sulphite any two so3 

Ans-   a) 1 mol of nitrogen atom = 14u = 14 g 

b) 4 moles of aluminium atom 4 x 27 =108 u 

10 moles of sodium sulphite = 10 x (  2 x 23 + 1 x 32+ 3 x 16) = 1260u 


Q8 . Convert into mole

a) 12 gram of Oxygen gas 

b)20 gram of water

c) 22 gram of carbon dioxide

Ans-  a) Given mass of Oxygen gas= 12:00 gram

 Molar mass of Oxygen gas  =32 gram

 Mole of Oxygen gas  12 /32 = 0.3 75 Mole 

b) Given mass of water 20 gram 

Molar mass of water 2 x 1 + 16 = 18 gram 

mole of water 20/18 =0.11 mole 

c) Given mass of carbon dioxide  - 22 gram

Molar mass of carbon dioxide 1 x 2 + 2 x 16 = 44 gram 

Mole of carbon dioxide 22/44= 0.5 mole 


Q9 . What is the mass of 

a) 0.2 mole of Oxygen atoms 

b) 0.5 moles of water molecules 

Ans-  a) Moles of oxygen atoms =0.2 mole 

Molar mass of oxygen atom   =16 gram 

Mass of oxygen atom 16  x 0.2  = 3.2gram 

b) Mole of water molecule =  0.5 mole 

Molar mass of water molecule 2 x 1 + 16  = 18 gram 

Mass of H2O =  18 x  0.5 = 9 gram. 


Q10 . Calculate the number of molecules of sulphur  (S8 ) present 16 gram of solid Sulphur 

Ans- Molar mass of Sulphur  = 256 g ( 6.0 22 x 10 23) 

Given mass of Sulphur =  16 gram 

Molecular mass of Sulphur 16  x 6.0 22  x 10 23 /  256 

      =  96.35x 10 23/       25

      =     0.376 x 10 23 molecules

                  

Q11. Calculate the number of Aluminium ions present in 0.051 gram of Aluminium oxide ( Hint- the mass of an ion is the same as that of an atom of the same element atomic mass of a l = 27 u ) 

Ans-  1 mole of Aluminium oxide Al2O3= 2 x 27+3 x 16= 102u 

102 g of Al2O3 has =  6.023 x 10 23 molecules 

0.0 51 gram of Al2O3 has =  6.022 x  10 23 x 0.051  /102

but one molecules of Al2O3 is = 2Al3+

   So,  2 x 3.01 x 10 20 = 6.023 x   10 20 Aluminium ion                                               

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