CBSE CLASS 9 CHEMISTRY

1. Molecular mass and
mole concept

2. WHAT IS MOLECULAR MASS??
 MOLECULAR MASS OF A SUBSTANCE IS THE SUM OF THE ATOMIC
MASSES OF ALL THE ATOMS IN THE MOLECULE OF THE SUBSTANCE.
 THEREFORE, THE RELATIVE MASS OF A MOLECULE IS EXPRESSED
IN ATOMIC MASS UNITS (u).
WHAT IS FORMULA UNIT MASS??
 THE FORMULA UNIT MASS OF AS SUBSTANCE IS THE SUM OF THE
ATOMIC MASSES OF ALL ATOMS IN A FORMULA UNIT OF A
COMPOUND.
 FOR EXAMPLE, SODIUM CHLORIDE HAS A FORMULA UNIT NaCl. IT’S
FORMULA UNIT CAN BE CALCULATED AS-
= 1*23+1*35.5=585u
THAT IS (no.of Na atoms*u of Na+ no.of Cl atoms*u of Cl)

3. WHAT IS THE MOLE CONCEPT??
 ONE MOLE OF ANY SPECIES(ATOMS, MOLECULES, IONS OR
PARTICLES) IS THAT QUANTITY IN NUMBER HAVING A MASS EQUAL
TO ITS ATOMIC OR MOLECULAR MASS IN GRAMS(g).
 THE NO.OF PARTICLES(ATOMS, MOLECULES OR IONS) PRESENT IN
ONE MOLE OF ANY SUBSTANCE IS FIXED, WITH A VALUE OF
6.022*10^23. THIS NUMBER IS CALLED AS THE AVAGRADO CONSTANT
OR AVAGRADO NUMBER.
 TO GET THE MASS OF 1 MOLE OF ATOM OF AN ELEMENT, THAT IS,
MOLAR MASS, WE HAVE TO TAKE THE SAME NUMERICAL VALUE BUT
CHANGE THE UNITS FROM “u” TO “g”.
 MOLAR MASS OF AN OBJECT IS ALSO KNOWN AS THE GRAM ATOMIC
MASS.
 Eg:- u OF hydrogen= 1u. SO, g OF hydrogen=1 g.
1u OF hydrogen HAS ONLY 1 ATOM OF hydrogen. 1g OF hydrogen
HAS ONLY
1 MOLE OF ATOMS,THAT IS, 6.022*10^23 ATOMS OF hydrogen.
 CHEMISTS NEED THE NO.OF ATOMS AND MOLECULES WHILE
CARRYING OUT REACTIONS, AND NEED TO RELATE MASS IN GRAMS
TO THE NUMBER. IT IS DONE AS FOLLOWS:
1 MOLE= 6.022*10^23 NUMBER

4. FACTS ABOUT THE MOLE CONCEPT!!
 THE WORD MOLE WAS INTRODUCED AROUND 1896 BY WILHELM
OSTWALD WHO DERIVED THE TERM FROM THE LATIN WORD MOLES
MAENING A HEAP OR PILE.
 THE SUBSTANCE MAY BE CONDIDERED AS A HEAP OF ATOMS OR
MOLECULES.
 THE UNIT MOLE WAS ACCEPTED IN 1967 TO PROVIDE A SIMPLE WAY
OF REPORTING A LARGE NUMBER– THE MASSIVE HEAP OF ATOMS
AND MOLECULES IN A SAMPLE.