JEE MANTRAMOLE CONCEPT

The concept of Atoms and Molecules

• Relative atomic mass of an element .Average mass per atom of the natural isotopic composition /1 /12 th mass of an atom of nuclide C-12

JEE MANTRA

 and •Relative Atomic Mass Calculation BROMINEbromine consists of 50%  7 9Br and 50%  8 1Br, calculate the Ar of bromine.A r  = [ (50 x 79) + (50 x 81) ] /100 = 80So the relative atomic mass of bromine is 80 or  RAM or A r(Br) = 80Note the full working shown.  Yes, ok, you can do it in your head BUT many students ignore the %'s and just average all the isotopic masses (mass numbers) given, in this case bromine-79 and bromine-81.

Relative molecular mass of the compound

• The ratio of average mass per molecule of the natural isotope to 1/12th mass of c-12,

• RMM (relative molecular mass)= sum of relative atomic masses of the atoms of molecule

Calculate the relative molecular mass of calcium hydroxide into the expression to

find Mr(  Ca3(PO4)3

• (Ca2+)3(PO43-)2 or Ca3(PO4)3, but it makes no difference to

the calculation of relative formula mass or relative molecular mass.

• atomic masses: Ca = 40, P = 31, O =16

• RFM or Mr = (3 x 40) + 3 x {31 + (4 x 16)} = (120) + (3 x 95) = 405

Atomic mass unit

• The quantity 1/12th mass of an atom C-12 is known as atomic mass unit

• 1 amu=1.66 *10^-24 gramsgrams1.660 538 782(83) × 10−24g 1 gram of C-12 = 6.02 x 10^23 amu The precise definition is that the atomic mass unit (u) is one twelfth of the mass of an isolated atom of carbon-12 (12C) at rest and in its ground state.[1] In other words, "A single atom of carbon-12 has a mass of 12 u exactly, by definition."

ATOMIC MASS

• The average mass per atom of natural isotopic composition is called atomic mass

• Atomic mass= Relative atomic mass x atomic mass unit

Molecular mass

• The average mass per molecule of natural isotopic composition of a compound is known as molecular mass

Molecular mass= Relative molecular mass x atomic mass unit

Mole

• Mole= Wt in gm atom/atomic wt