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NCERT Solution, MCQs, Study Notes, Q & A By JSUNIL Q.1:  The average atomic mass of a sample of   an element X is 16.2 u. What is the % of isotopes 16 8 X     and   18 8 X in the sample? Ans:  Let % of isotopes 16 8 X   is y then   18 8 X in the sample is 100-y 16.2 = y% of 16 + (100-y)%of18 Þ 16.2 x100 = 16y+1800-18y Þ 1620-1800 =-2y Þ 180=2y Þ y = 90% % of isotopes 16 8 X   is y =   90%   then 18 8 X in the sample is 100-y =100-90=10% Q2: Why is it not possible to see an atom with naked eyes? Answer : The size of an atom is so small that it cannot be seen by naked eye or by simple optical instruments. e.g. Considering an atom a sphere, the radius of Hydrogen atom is approximately 10nano-metres (1 nm = 10 -9 m). To see atoms of different elements, a special type of microscope called scanning tunnelling microscope  (STM) is used. Q3: Name the element having following Latin names (i)   Stibium          (ii)  Cuprum        (iii) Argentum 

1. Avogadro’s number represents the number of atoms in (a) 12g of C 12                                 (b) 320g of sulphur       (c) 32g of oxygen                            (d) 12.7g of iodine 2. The number of moles of carbon dioxide which contain 8 g of oxygen is (a) 0.5 mol                                  (b) 0.20 mol                               (c) 0.40 mol                                (d) 0.25 mol (3) The total no of ions present in 111 g of cacl2 is (a) One mole                               (b) Two mole               (c) Three mole                                (d) Four moles (4) Which of the following weighs the most ? (a) one g-atom of nitrogen            (b) One mole of water (c) One mole of sodium                (d) One molecule of H2SO4 (5) 5.0 litre of 0.4 M H2SO4 Contains- (a) 2.0 Mole Of H2So4                  (b) 0.4 mole H2So4 (c) 5.0 mole H2So4                       (d) 2.0 moles H2O Ans: (1) a  (2) d (3) c (4) c (5) a

CBSE Physics help class 8 Chapter: Sound 1. What is the difference between noise and music? Answer:     Unpleasant sounds are called noise whereas music is a sound, which produces pleasant sensation. But, if the musical sound is too loud, it becomes noise. 2. What is eardrum? How does it play an important role in hearing? Answer:     A thin tightly stretched membrane t is called eardrum. it vibrates sound wave and sends vibrations to the inner ear. 3.  Which property of vibrations determines the pitch of the sound? Explain giving some examples. Answer:     The frequency determines the shrillness or pitch of a sound. If the frequency of vibration is higher the sound has a higher pitch. If the frequency of vibration is lower, the sound has a lower pitch. Women sound is shriller than man due to high frequencies 4. What is the audible range of frequencies for human beings? Answer:     The range of audible frequencies is roughly between 20 to 20,000

Q.1. Can any object have mechanical energy even if its momentum is zero? Explain. Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there it no kinetic energy but the object may possess potential energy. Q.2. The potential energy of a body is 39600J. How high is the body if its mass is 20kg? The potential energy of a body = mgh h = PE/mg = 39600j/20kg x9.8m/s 2 =198m Q.3. A force of 20 N displaces a body through a distance of 1 m at an angle of 60° from its own direction. Calculate the amount of work done. Here, force F = 20 N, displacement, s = 1 m. Angle between force and displacement 60°. Work done,W =Fscos0 =20 X 1 X cos60°=20X 1 X 1/2 = 10J. A man of 50 kg jumps up to a height of 1.2 m. What is his potential energy at the highest point? The potential energy of man = mgh = = 50 + 10 X 1.2 J = 600 J Q.4. How much work is done by a force of 10 N in moving an object through a distance o

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