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Thursday, August 25, 2022

Atomic mass:

(1) Atomic mass:
Atomic mass represent the mass of single isotopes or single atoms.

"Atomic mass and atomic weight do not mean the same:

Atomic mass is equal to the mass of total number of protons , neutrons  and electrons ( mass of electrons are negligible hence ignore)

Atomic mass = total mass of nucleons (total number of nucleons × mass of ons nucleons.

Atomic mass is independent of the atomic masses of isotopes.

Calculated  value of the atomic mass is whole number

Unit of atomic mass: the unit of atomic mass is atomic mass unit (amu) or unified atomic mass (u) and amu also known as Dalton(Da)

It is representation of unit of atomic weight is a.m.u. and  it is the smallest unit of mass and is define as;

We know that one mole (6.022x10^23) atoms C^12 atoms has  12 gm mass.

Hence mass of one C^12 atom is = 12/N_A=  12/6.022x 10^23 gm =1.99 x 10^-23 gm.

 The real mass of one atom of C^12 has been determined as 1.9924 × 10^-23 gram.

Conclusively we can summarise as

1 amu is = mass of one nucleons (protons or  nutrons)

1 amu is = 1.67×10^27 kg 

1 amu is = mass of one proton or nutron ( 1.67×10^27 kg )

(2) Atomic weight or relative average Atomic mass:

(3) Gram-atomic weight: When atomic weights of elements are expressed in grams, they are called gram atomic weights. 

Thus, the weight of an element in grams which is equal to its atomic weight is known as the gram atomic weight (GAW) of that element. 

  For example, atomic weight of sodium is 23 and its gram atomic weight is 23 grams. Similarly, the gram atomic weights of hydrogen, sulphur, nitrogen are 1.008 gram, 32 gram and 14 gram respectively.


1 GAW P = 31.0 gram phosphorus,

1 GAW K = 39.0 gram potassium,

1 GAW Ca = 40.0 gram calcium.

 (4) Gram atom or mole atom: The atomic weight of an element taken in grams is one gram-atom or mole atom of that element. 

For instance:

one gram atom of oxygen represents 16 gram oxygen.

One gram atom of sulphur represents 32 gram sulphur. 

Number of gram atoms of an element

  "One gram atomic weight of every element contains 6.023 × 10^23 atoms of the element.”

Other Questions:

(1) Calculate the average atomic weight of an element (X) containing three isotopes X56, X57 and X59 and natural occurrence of these isotopes are 90%, 8% and 2% respectively.

(2) For the reaction Initially 2.5 moles of Fe(NO3)3 and 3.6 moles of Na2CO3 are taken. If 6.3 moles of Na2CO3 is obtained the % yield of given reaction is ?

(3) 3.9 gm Al(OH)3 is reacted with excess of HCl , 6.5 gm AlCl3 is formed determined % yield of reaction ?

(4) 200 gm of CaCO3 on heating produces 11.2 litre of CO2 (g) at STP calculate % yield of reaction?

(5) How many grams of Fe2O3 formed by heating of 18 gm FeO with O2.

(6) How many moles O2 are required to needed produced 5 moles of Fe2O3.

(7) 367.5 gm KClO3 (122.5) when heated then calculate. (1) How many gram of O2 is produced? (2) How Many litre of O2 is produced at STP?

(8) Air contain 20 % O2 by volume. How many cm³ of air will be required for oxidation of 100 cm³(ml) of acetylene?

(9) The hydrated salt, Na2SO4.nH2O undergoes 55.9% loss in weight on heating and becomes anhydrous. The value of n will be?

(10) Haemoglobin contains 0.33% of Iron by weight. The molecular weight of it is approx. 67200. The numbers of iron atoms (Atomic wt of Fe=56 u) present in one molecule of Haemoglobin are?

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