## Saturday, October 20, 2012

### 9th Atoms and molecules NCERT Solution, MCQs, Study Notes, Q & A

1. Q. While searching for various atomic mass units, scientists initially took 1/16 of the mass an atom of naturally occurring oxygen as one unit. State two reasons for this.

Ans: initially, scientist chose Hydrogen as a slandered substance to measure relative mass of element because it it lightest of all elements but latter oxygen was fix because it is more reactive than hydrogen
Two reasons for this are:
(1)Oxygen reacts with large number of elements and form compounds
(2)This atomic mass gives masses of elements in whole number
But latter on it was observed that atoms of natural occurring oxygen has different masses (called isotopes of oxygen) than in 1961 IUPAC selected stable isotopes of carbon ( carbon-12) as the slandered for comparing the atomic mass or molecular mass.

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.

2.Q.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.3. How do we know the presence of atoms if they do not exist independently for most of the elements?
Answer: Atom join in different way to form matter(neutral molecules or ion) that we are able to touch, feel and see.

Q.4.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%
Q.5. (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.
Ans: (a) J. J. Berzelius (b) The symbol of an element is the ‘first letter’ or the ‘first letter and another letter’ of the English name or Latin name of the element.
Sodium has Latin name Natrium and so the symbol is Na
(c) oxygen = o2          ozone = o3
Related post to IX(9th)  Atoms and Molecules