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## Saturday, October 20, 2012

### IX(9th) Atoms and Molecules Questions Answers By Jsunil

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 168X   and  18 8X in the sample?
Ans: Let % of isotopes 168X   is y then  18 8X in the sample is 100-y
16.2 = y% of 16 + (100-y)%of18
Þ 16.2 x100 = 16y+1800-18y
Þ1620-1800 =-2y
Þ 180=2y
Þy = 90%
% of isotopes 168X   is y =  90%  then 18 8X 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 = 10-9m).
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           (iv) Natrium
(v)  Stannum       (vi) Wolfram             (vii) plumbum           (viii) Kalium
(i)   Stibium - Antimony (Sb)                           (ii)  Cuprum - Copper (Cu)
(iii) Argentum - Silver(Ag)                              (iv) Natrium - Sodium (Na)
(v)  Stannum - Tin (Sn)                                    (vi) Wolfram - Tungeston (W)
(vii) plumbum - Lead (Pb)                                (viii) Kalium - Potassium (K)
Q4: What is the significance of a chemical symbol?
Ans: The symbol of an element has the following significance:
1. Symbol stands for the name of the element in while representing chemical formula and chemical equations.
2. it stands for one atom of the element.
3. it represents quantity of the element in terms of mass, moles or gram-atomic mass etc. while computing chemical reactions.
Q5: Why do atoms of the most of the elements not exist independently?
Answer: Atoms of the most of the elements are reactive by nature. They are not able to exist independently and form molecules and ions.
Q6: What is the atomic mass unit?
Answer: The mass equal to 1/12th of the mass of  126C atom is called the one atomic mass unit. It is 1.66 × 10-27 kg.  It is represented by symbol 'u' (older symbol 'amu').
1 u  = 1/12th of the mass of carbon-12 atom.

Q7: Magnesium is two times heavier than C-12 atom, what shall be the mass of Mg atom in terms of atomic mass units? (Given mass of C-12 atom = 12u)
Answer:  1 u = 1/12th of C-12 atom      mass of C-12 atom = 12u
Mass of Mg is twice of C-12 atom = 2 ×12u  = 24u
Q8. What is relative atomic mass  of an element? How it is related to atomic mass unit?
Ans: The relative atomic mass (Ar) of an element is the average relative mass of an element of the element with respect to an atom of Carbon-12 taken as 12u.

Average mass of 1 atom of an element
Relative Atomic mass of an element = --------------------------------------
1/12th of Mass of one C-12 atom

The relative atomic mass is a pure number and has no units. It indicates number of times one atom of an element is heavier than 1/12th of C-12 atom.
Relative atomic mass is also called average atomic mass
Atomic mass is expressed in amu or u while relative atomic mass is a number.

Q.9. Give the names of the elements present in the following compounds.
(a) Quick lime (b) Hydrogen bromide  (c) Baking powder (d) Potassium sulphate.
(a) Quick lime: Calcium Oxide (CaO), Elements Present: Calcium(Ca), Oxygen(O)
(b) Hydrogen bromide: HBr, Elements Present: Hydrogen (H), Bromine(Br)
(c) Baking powder: Sodium Bicarbonate (NaHCO3Elements Present: Sodium(Na), Hydrogen(H), Carbon(C) and Oxygen(O).
(d) Potassium sulphate: K2SO4Elements Present:Potassium(K), Sulphur(S), Oxygen(O)
Q.10. What are the postulates of Dalton's atomic theory?
AnsThe postulates of Dalton's atomic theory are:
1. All matter is made of very tiny particles called atoms.
2. Atoms are indivisible particles i.e. can neither be created or destroyed in a chemical reaction. (Law of Conservation of mass)
3. Atoms of a give element have same mass and identical chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds. A chemical reaction is a re-arrangement of atoms. (Law of Multiple Proportions)
6. The relative number and kinds of atoms are constant in a given compound. (Law of Definite Proportions)
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