MOLECULAR MASS

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 = Relative mass in grams Thus, a mole is a chemist’s unit

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.