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## Wednesday, February 29, 2012

### IX Structure of atoms Solved Exam accelerator important questions

Q. A naturally occurring sample of :-
(i) 69.2% of 53Cu& 30.8% of 65Cu. Find the average atomic mass of a naturally occuring sample of copper.
(ii)7.42% of 6Li & 92.58% of 7Li.Find the average atomic mass of a naturally occuring sample of Lithium.

Ans:  Average atomic mass of naturally occurring copper = (53 x 69.2)/00 + (65 x 30.8)/100
= 3667.6 + 2002 / 100 =  5669.6/100 =56.696 u
Average atomic mass of naturally occurring lithium = 6 x 7.42 + 7 x 92.58 /100
= 44.52 + 648.06 /100         = 692.58/100 = 6.9258 u

Q. Calculate the no. of atoms of each element persent in 9.8 g of sulphuric acid, H2SO4. (H=1, S=32, O=16)

Ans: Molecular weight of H2SO4 = 98
9.8 gm of H2SO4 = 9.8/98 = 0.1mole
1 mole = 6.023 x 1023 molecules
0.1 Mole = 6.023 x 1022 molecules.
One molecule of H2SO4 = 2 hydrogen atom, one sulphur atom and 4 oxygen atoms therefore
Number of hydrogen atom in 0.1 mole = 2 x 6.023 x 1022= 12.046 x 1022
Number of sulphur atom in 0.1 mole = 1 x 6.023 x 1022= 6.023 x 1022
Number of oxygen atom in 0.1 mole = 4 x 6.023 x 1022= 24.092 x 1022

Q. How to calculate the atomicity and the atomic mass of an atom?

Ans: Atomicity may be defined as number of atoms present in one molecule. For example N2 is diatomic; P4 is tetra atomic in nature.

Atomic Mass of an atom is the sum of proton and neutron in the nucleus. For example the atomic mass of sodium is 23 because it contain 11 proton and 12 neutron.

Q.To  weight BaCl2 or Na2SO4
a) use a polythene bags and spring balance              b) use a watch glass and spring balance
c) use a polythene bags and physical balance          d) use a watch glass and physical balance

Ans: For this use a watch glass and physical balance.
First weigh the watch glass then put salt on the watch glass and weigh.
The weight of the salt = (weight of watch glass+ salt ) - ( weight of watch glass)

Q.To weight sodium sulphate or barium chloride it should be in form of
a) saturated solution    b) large crystals         c) small crystals          d) fine powder

Q.  what is the distribution of electrons in an atom of Phosphorus and how can it have two valencies

Ans: phosphorus has large atomic size and therefore posses excess electron bearing capacity in its shell, therefore it can share 3 electrons and 5 electrons to form trivalent and penta valent compounds.
also,  Phosphorus can expand its valency due to presence of 3d sub-shell hence shows trivalent and pentavalent nature. For example  PCl3 and PCl5.

Or,
Atomic number of phosphorus is 15, therefore the electronic configuration is 2 10 5
Phosphorus has tendency to expand its valency from 3 to 5 due to large size and available orbitas therefore exist in PCl3 and PCl5
Q.The maximum no. of electrons present in shell is given by the formula 2n2and the maximum no. of electrons filled in M shell is 18. But in the element calcium we only fill 8 electrons in the M shell and move on to the N shell. why ?

Ans: According to Bohr burry rule, the number of electron exceeds 8 when it gives 2 electrons to next level shell. In Calcium, the number of electron cannot exceed more than 8 in M shell until it gives 2 electrons to next level. Therefore the electronic configuration of calcium is  20Ca : 2 8 8 2

Q. What are alpha particles?

Ans: Alpha particle is nucleus of helium, it carry +2 charge therefore has 2 proton and 2 neutron.
2He4  alpha + 2e

Q. Write ddifference between atomic mass and mass number?

Ans: Atomic mass is the average mass of element generated by taking the average of all the isotopes present in the nature. For example atomic mass of carbon is 12.01.

Mass number defines the total number of proton and neutron present in the nucleus of an atom. For example Mass number of carbon is 12
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Q. An ion  (M2+) contain 10 electrons and 12 neutrons what is the atomic number and mass number of the element M.
Ans: Since, M2+ contains 10 electrons and the M2+ ion suggests that it has lost 2 electrons so it contains total 12 electrons. The numbers of electrons are equal to number of protons in an atom, so its atomic number is 12.
The no of protons and neurons is known as mass number which is 24 (12 + 12 = 24.)

Q. an ion M3+ contains 10 electrons and 14 neutrons.What are the atomic mass and mass number of the element M ?  Name the element
Ans: The atomic number of element is "13" and the atomic mass is "13+14=27u", the name of the element is "aluminium".

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#### 1 comment:

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