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## Thursday, October 15, 2015

### 9th CBSE Science Chap 3 Atoms and Molecules Self study Questions

Solved problems:Class 9 _Atoms and Molecules

1. Q. 5 g of calcium combine with 2 g of oxygen to form a compound. Find the molecular formula of the compound. (Atomic mass of Ca = 40 u; O = 16 u)

Ans. Number of moles in 5g of calcium = mass / molar mass    =5/40    =  0.125
Number of moles in 2g of oxygen = mass / molar mass         =  (2 / 16)  =  0.125

Now we will calculate the simplest ratio of the element by dividing the number of moles of each element by the smallest value.

Since number of moles of each element is 0.125, therefore calcium and oxygen are present in a ratio of 1 : 1. Thus the empirical formula of the compound is CaO.

For calculating the molecular formula, we need the molecular mass of the compound. However, because a compound with the formula CaO is known,

Therefore the molecular formula of the compound is CaO.

. Q. (i) Name the body which approves the nomenclature of elements and compounds.
(ii) The symbol of sodium is written as Na and not as S. Give reason.
(iii) Name one element which form diatomic and one which form tetra atomic molecules. [CBSE 2011 ]

Ans. (i) IUPAC (International Union of Pure and Applied Chemistry)
(ii) Latin name of sodium is Natrium. The first two letters (Na) of this name represents the symbol of sodium.
(iii) Oxygen forms diatomic molecules and phosphorus forms tetra atomic molecules.

3. Q. Calcium and Oxygen are combined in the rates of 5:4 by mass to form calcium oxide.  What mass of Oxygen gas would be required to react with 2.5 g of calcium ?

Ans:  Calcium and oxygen combine in the rates of 5:4 by mass to form calcium oxide
let x gram of oxygen is required to react with 2.5 g of calcium to form calcium oxide
Therefore 5/4 = 2.5 / x

5 x = 2.5 x  4
x = (2.5 x 4 ) / 5
So, x = 2
Therefore 2 grams of oxygen is required to react with 2.5 grams of calcium to form calcium oxide .

Problems (based on mole concept)

4.Q. a. Calculate the number of moles in 81g of aluminium

Solution : Atomic mass of Al= 27gm
27g  of  aluminium = 1 mole of aluminium
81g  of aluminium   = 1/27 x 81=3 moles of aluminium
OR, Use formula , Number of moles = given mass/atomic mass
Self: ii) 4.6g sodium   iii) 5.1g of Ammonia  iv) 90g of water v) 2g of NaOH

5. Q. Calculate the mass of 0.5 mole of iron

Solution: Atomic mass of iron = 55.9 g
Mass of the 1 mole of iron   = 55.9 g
Mass of the 0.5 mole of iron   = 0.5 x 55.9 g = 27.95 g
Or, Using formula: mass = atomic mass x number of moles
FOLLOW UP: Find the mass of 2.5 mole of oxygen atoms

6.Q.Calculate the number of molecules in 11g of CO2

Solution: gram molecular mass of CO2 = 44g
44g of CO2  = 6.023 x 1023  molecules
1 g of CO2  = ( 6.023 x 1023 ÷ 44 g ) molecules
11g of CO= ( 6.023 x 1023 ÷ 44 g ) x 11 = 1.51 x 1023 molecules
FOLLOW UP: Calculate the number of molecules in 360g of glucose

7. Q. Calculate the mass of glucose in 2 x 1024 molecules

Solution: Gram molecular mass of glucose = 180g
Mass of glucose  [180 x 2 x 1024 ] / [6.023 x 1023  ] = 597.7g
FOLLOW UP: Calculate the mass of 12.046 x 1023 molecules in CaO.

8. Q. Calculate the number moles for a substance containing 3.0115 x 10^23 molecules in it.
Solution: Number of moles = Number of molecules/Avogadro Number

= [3.0115 x 1023 ] / [6.023 x 1023  ]      = 0.5 moles

9. Q. Calculate the mass of 18.069 x 1023 molecules of SO2
Solution:
Mass of a substance =  gram molecular mass x number of particles/ 6.023 x 1023
Gram molecular mass SO2 = 64g
6.023 x 1023 molecules of SO2     = 64 gm
1           molecules of SO2     = 64/(6.023 x 1023 )  gm
18.069 x 1023  molecules of SO2   = [ 64/(6.023 x 1023 )  x  18.069 x 1023  ]gm = 192 g

Class 9 Atoms and molecules NCERT Solutions-1

Class 9 Atoms and molecules NCERT Solutions-2

Solved Problems (based on mole concept)

Chapter based Multiple Choice Questions

Class 9 SA2: Atoms and molecules solved CBSE Test Papers New

Class 9 Atoms and molecules solved CBSE Test Paper-1

Class 9 Atoms and molecules solved CBSE Test Paper-2

Class 9 Atoms and molecules solved CBSE Test Paper-3

Class 9 Atoms and molecules solved CBSE Test Paper-4