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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

Other Download study links:  Chap 3 Atoms and Molecules class9

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

Class 9 Atoms and molecules solved CBSE Test Paper-5         Download Files

## Friday, October 2, 2015

### Floatation_Class-9 important solved Questions from previous year CBSE Board Paper

Fully Solved Questions from previous year board Paper
Question: 1. What causes buoyant force (or up thrust) on a boat?

Answer: Buoyant force is due to volume of boat and density of water.

Question: 2. what is the cause of buoyant force?

Answer: : The pressure difference on lower part and upper part of a body cause of buoyant force .

Question: 3. An Object of volume V is immersed in a liquid of density P. Calculate the magnitude of buoyant force acting on the object due to the liquid

Question: 4. why does a block of wood held under water rise to the surface when released?

Answer: This happens because as upward force is greater than the weight.

Question: 5. What happens to the buoyant force as more and more volume of a solid object is immersed in a liquid. When does the buoyant force become maximum?

Answer: The buoyant force exerted by a fluid is equal to the weight of fluid displaced. Therefore the buoyant force increases as more and more volume of a solid object is immersed in a liquid.
Buoyant force is maximum when a body is completely immersed in a fluid.

Question: 6. Explain why, a piece of glass sinks in water but it floats in mercury.

Answer: The density of glass is more than water, but less then mercury, Therefore glass float on mercury and sinks in water.

Question: 7. When is the pressure on the ground more when a man is walking or when a man is standing ? Explain.

Answer :  When a man is walking is weight is alternately on one feet or the other, when he is standing, his weight is divided on both feet. Therefore pressure is more on the ground when he is walking (as surface area is less).

Question: 8.  A solid weighs 80 g in air, 64 g in water. Calculate the relative density of solid. When kept in water, state if the object would float or sink?

Answer:  Weight loss in water = (80 -  64) g = 16
Relative density of solid = (weight in air/ Weight loss in water) = 80/16= 5
Since relative density of water is 1 which is less than that of solid. Hence Solid Sink

Question: 9. why density varies?

Answer: The density of a material depends on two things.
(i) The individual mass of each atom or molecule of substance  (ii) How tightly the atoms are packed

Question: 10. An object is suspended with a string . The string is stretched. When the object is completely immersed in water, the extension of the thread decreases. Why?

Answer : This happen because when the object is completely immersed in water it experience an upward force that cause weight loss.
More Questions are solved Here:  http://jsuniltutorial.weebly.com/
Sure shoot MCQ for class 9 science term-2 ClicK Here